* process.c: Add Process._fork

This API is supposed for application monitoring libraries to hook fork
event.

[Feature #17795]

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

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

Allow the 'Dir.home' method to reliably locate the user's home directory when
all three of the following are true at the same time:

    1. Ruby is running on a Unix-like OS
    2. The $HOME environment variable is not set
    3. The process is not a descendant of login(1) (or a work-alike)

The prior behavior was that the lookup could only work for login-descended
processes.

This is accomplished by looking up the user's record in the password database
by uid (getpwuid_r(3)) as a fallback to the lookup by name (getpwname_r(3))
which is still attempted first (based on the name, if any, returned by
getlogin_r(3)).

If getlogin_r(3), getpwnam_r(3), and/or getpwuid_r(3) is not available at
compile time, will fallback on using their respective non-*_r() variants:
getlogin(3), getpwnam(3), and/or getpwuid(3).

The rationale for attempting to do the lookup by name prior to doing it by uid
is to accommodate the possibility of multiple login names (each with its own
record in the password database, so each with a potentially different home
directory) being mapped to the same uid (as is explicitly allowed for by
POSIX; see getlogin(3posix)).

Preserves the existing behavior for login-descended processes, and adds the
new capability of having Dir.home being able to find the user's home directory
for non-login-descended processes.

Fixes [Bug #16787]

Related discussion:
    https://bugs.ruby-lang.org/issues/16787
    https://github.com/ruby/ruby/pull/3034

To fix build failures.

Devs do not love "3".  The only exception is RUBY3_KEYWORDS in parse.y,
which seems unrelated to our interests.

This shall fix compile errors.

As fork(2) is deprecated, its calls must be guarded by
`COMPILER_WARNING_IGNORED(-Wdeprecated-declarations)`.
All usages of fork(2) in process have been alread guarded.  A new call
to fork(2) was added in ruby.c with f22c4ff359498ab342e4b6d6feb21af6004ee270.
This caused a build failure on Solaris 11.

It may hide a bug to guard big code unnecessarily, so this change
introduces a simple wrapper "rb_fork" whose definition is guarded, and
replaces all calls to fork(2) with the wrapper function.

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