This reverts commit 35136e1e9c232ad7a03407b992b2e86b6df43f63.

test-spec has been failing since this revision.

.github/workflows/compilers.yml:82
https://github.com/ruby/ruby/actions/runs/4276884159/jobs/7445299562

```
env:
  # Minimal flags to pass the check.
  default_cc: 'gcc-11 -fcf-protection -Wa,--generate-missing-build-notes=yes'
  optflags: '-O2'
  LDFLAGS: '-Wl,-z,now'
  # FIXME: Drop skipping options
  # https://bugs.ruby-lang.org/issues/18061
  # https://sourceware.org/annobin/annobin.html/Test-pie.html
  TEST_ANNOCHECK_OPTS: "--skip-pie --skip-gaps"
```

Failure:

```
  1)
  An exception occurred during: Kernel#require (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:317
  Kernel#require (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.ext.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:5:in `<top (required)>'

  2)
  An exception occurred during: Kernel#require ($LOADED_FEATURES) stores an absolute path
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:330
  Kernel#require ($LOADED_FEATURES) stores an absolute path ERROR
  LeakError: Closed file descriptor: 8
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:5:in `<top (required)>'

  3)
  An exception occurred during: Kernel#require ($LOADED_FEATURES) does not load a non-canonical path for a file already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:535
  Kernel#require ($LOADED_FEATURES) does not load a non-canonical path for a file already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:5:in `<top (required)>'

  4)
  An exception occurred during: Kernel#require ($LOADED_FEATURES) does not load a ../ relative path for a file already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:551
  Kernel#require ($LOADED_FEATURES) does not load a ../ relative path for a file already loaded ERROR
  LeakError: Leaked file descriptor: 9 : #<File:../code/load_fixture.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:5:in `<top (required)>'

  5)
  An exception occurred during: Kernel#require ($LOADED_FEATURES) complex, enumerator, rational, thread, ruby2_keywords are already required
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:563
  Kernel#require ($LOADED_FEATURES) complex, enumerator, rational, thread, ruby2_keywords are already required ERROR
  LeakError: Closed file descriptor: 8
  Closed file descriptor: 9
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:5:in `<top (required)>'

  6)
  An exception occurred during: Kernel.require (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:317
  Kernel.require (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.ext.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:23:in `<top (required)>'

  7)
  An exception occurred during: Kernel.require ($LOADED_FEATURES) stores an absolute path
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:330
  Kernel.require ($LOADED_FEATURES) stores an absolute path ERROR
  LeakError: Closed file descriptor: 8
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:23:in `<top (required)>'

  8)
  An exception occurred during: Kernel.require ($LOADED_FEATURES) does not load a non-canonical path for a file already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:535
  Kernel.require ($LOADED_FEATURES) does not load a non-canonical path for a file already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:23:in `<top (required)>'

  9)
  An exception occurred during: Kernel.require ($LOADED_FEATURES) does not load a ../ relative path for a file already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:551
  Kernel.require ($LOADED_FEATURES) does not load a ../ relative path for a file already loaded ERROR
  LeakError: Leaked file descriptor: 9 : #<File:../code/load_fixture.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:23:in `<top (required)>'

  10)
  An exception occurred during: Kernel.require ($LOADED_FEATURES) complex, enumerator, rational, thread, ruby2_keywords are already required
  /__w/ruby/ruby/src/spec/ruby/core/kernel/shared/require.rb:563
  Kernel.require ($LOADED_FEATURES) complex, enumerator, rational, thread, ruby2_keywords are already required ERROR
  LeakError: Closed file descriptor: 8
  Closed file descriptor: 9
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_spec.rb:23:in `<top (required)>'

  11)
  An exception occurred during: Kernel#require_relative with a relative path (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:197
  Kernel#require_relative with a relative path (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.ext.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:4:in `<top (required)>'

  12)
  An exception occurred during: Kernel#require_relative with a relative path ($LOADED_FEATURES) stores an absolute path
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:205
  Kernel#require_relative with a relative path ($LOADED_FEATURES) stores an absolute path ERROR
  LeakError: Closed file descriptor: 8
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:4:in `<top (required)>'

  13)
  An exception occurred during: Kernel#require_relative with an absolute path (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:399
  Kernel#require_relative with an absolute path (file extensions) does not load a C-extension file if a complex-extensioned .rb file is already loaded ERROR
  LeakError: Leaked file descriptor: 8 : #<File:/__w/ruby/ruby/src/spec/ruby/fixtures/code/load_fixture.ext.rb>
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:277:in `<top (required)>'

  14)
  An exception occurred during: Kernel#require_relative with an absolute path ($LOAD_FEATURES) stores an absolute path
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:407
  Kernel#require_relative with an absolute path ($LOAD_FEATURES) stores an absolute path ERROR
  LeakError: Closed file descriptor: 8
  /__w/ruby/ruby/src/spec/ruby/core/kernel/require_relative_spec.rb:277:in `<top (required)>'
```

When loading Ruby source files, we can save the result of
successful opens as open(2)/openat(2) are a fairly expensive
syscalls.  This also avoids a time-of-check-to-time-of-use
(TOCTTOU) problem.

This reduces open(2) syscalls during `require'; but should be
most apparent when users have a small $LOAD_PATH.  Users with
large $LOAD_PATH will benefit less since there'll be more
open(2) failures due to ENOENT.

With `strace -c -e openat ruby -e exit' under Linux, this
results in a ~14% reduction of openat(2) syscalls
(glibc uses openat(2) to implement open(2)).

 % time     seconds  usecs/call     calls    errors syscall
 ------ ----------- ----------- --------- --------- ----------------
   0.00    0.000000           0       296       110 openat
   0.00    0.000000           0       254       110 openat

Additionally, the introduction of `struct ruby_file_load_state'
may make future optimizations more apparent.

This change cannot benefit binary (.so) loading since the
dlopen(3) API requires a filename and I'm not aware of an
alternative that takes a pre-existing FD.  In typical
situations, Ruby source files outnumber the mount of .so
files.

These contents are purely implementation details, not worth appearing in
CAPI documents. [ci skip]

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

Split ruby.h

These headers need no rewrite.  Just add some minor tweaks, like
addition of #include lines.  Mainly cosmetic.

TIMET_MAX_PLUS_ONE was deleted because the macro was used from only
one place (directly write expression there).

Reduce macros for readability.  Also transplanted some part of
internal/file.h into here because the delcared functions are in fact
defined in error.c.

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.