On Win32, currently, rb_nativethread_cond_t is an incomplete type
because it's a typedef for `struct rb_thread_cond_struct`. That means
you can't actually allocate a rb_nativethread_cond_t unless you also
include THREAD_IMPL_H (since its defined in thread_win32.h)
(alternatively, including vm_core.h also works).

Move the definition of rb_thread_cond_struct into thread_native.h to
alleviate this.

This implementation does nothing around preemptive context switching
because there is no native thread.

Must not be a bad idea to improve documents. [ci skip]

To make some kind of Ractor related extensions, some functions
should be exposed.

* include/ruby/thread_native.h
  * rb_native_mutex_*
  * rb_native_cond_*
* include/ruby/ractor.h
  * RB_OBJ_SHAREABLE_P(obj)
  * rb_ractor_shareable_p(obj)
  * rb_ractor_std*()
  * rb_cRactor

and rm ractor_pub.h
and rename srcdir/ractor.h to srcdir/ractor_core.h
    (to avoid conflict with include/ruby/ractor.h)

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

  [Feature #9612]
* ext/openssl/ossl.c (ossl_fips_mode_set): ditto.
* ext/coverage/depend: ditto.
* include/ruby/thread_native.h: added.
  This header file only provides wrapper functions to control
  native threads. These wrapper functions are used by MRI
  implementation.
* vm_core.h: use include/ruby/thread_native.h.
* thread.c: ditto.
* thread_pthread.h: ditto.
* thread_win32.h: ditto.
* thread_native.h: removed.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45944 b2dd03c8-39d4-4d8f-98ff-823fe69b080e