Redirect `rb_nogvl` blocking operations to the fiber scheduler if possible
to prevent stalling the event loop.

[Feature #20876]

This does not change any actual behaviour, but provides a choke point for blocking IO operations.

* Update `IO::Buffer` to use `rb_io_blocking_region`.

* Update `File` to use `rb_io_blocking_region`.

* Update `IO` to use `rb_io_blocking_region`.

duplicated typedef declaration was not allowed in C99.

* Add deprecations for public struct rb_io members.

This reverts commit 18e55fc1e1ec20e8f3166e3059e76c885fc9f8f2.

fix [Bug #19704]
https://bugs.ruby-lang.org/issues/19704
This breaks compatibility for extension libraries. Such changes
need a discussion.

* Add rb_io_path and rb_io_open_descriptor.

* Use rb_io_open_descriptor to create PTY objects

* Rename FMODE_PREP -> FMODE_EXTERNAL and expose it

FMODE_PREP I believe refers to the concept of a "pre-prepared" file, but
FMODE_EXTERNAL is clearer about what the file descriptor represents and
aligns with language in the IO::Buffer module.

* Ensure that rb_io_open_descriptor closes the FD if it fails

If FMODE_EXTERNAL is not set, then it's guaranteed that Ruby will be
responsible for closing your file, eventually, if you pass it to
rb_io_open_descriptor, even if it raises an exception.

* Rename IS_EXTERNAL_FD -> RUBY_IO_EXTERNAL_P

* Expose `rb_io_closed_p`.

* Add `rb_io_mode` to get IO mode.

---------

Co-authored-by: KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>

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