On glibc O_CLOEXEC is defined in fcntl.h and not unistd.h so this change
prevents the macro from being redefined.

Delete duplicated `Document-const:` directives

Those directives are attached to `rb_define_const` and `rb_file_const` where we don’t need them because the functions are supported by RDoc.

Raise SystemCallError exception when these functions return an error.

This changes behavior for the following case (found by the tests):

```ruby
dir1 = Dir.new('..')
dir2 = Dir.for_fd(dir1.fileno)
dir1.close
dir2.close
```

The above code is basically broken, as `dir1.close` closed the file
descriptor.  The subsequent `dir2.close` call is undefined behavior.
When run in isolation, it raises Errno::EBADF after the change, but
if another thread opens a file descriptor between the `dir1.close`
and `dir2.close` calls, the `dir2.close` call could close the file
descriptor opened by the other thread.  Raising an exception is much
better in this case as it makes it obvious there is a bug in the code.

For the readdir check, since the GVL has already been released,
reacquire it rb_thread_call_with_gvl if an exception needs to be
raised.

Due to the number of closedir calls, this adds static close_dir_data
and check_closedir functions.  The close_dir_data takes a
struct dir_data * and handles setting the dir entry to NULL regardless
of failure.

Fixes [Bug #20586]

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>

Fixes [Bug #20587]

* Release GVL for fdopendir calls
* Release GVL for readdir calls
* Release GVL for chdir call in dir_chdir0
* Release GVL for fchdir call in dir_fchdir
* Release GVL for chroot calls
* Release GVL for lstat calls
* Release GVL for stat calls
* Release GVL for fstatat calls
* Release GVL for getpwnam call in rb_home_dir_of
  (technically in file.c, but called from dir.c)

This does not release GVL for readdir/stat/lstat on Windows,
as that causes issues because the emulation functions that
are called in win32.c require the GVL.

This also removes some explicit casts either to or from void *,
which are allowed implicitly.  The remaining casts to or from
void * are part of function pointer casts, which are not
allowed implicitly and will generate a warning.

s/Retruns/Returns/

Ref: https://github.com/ruby/ruby/pull/10872

These should be the last internal uses of the old `Data` API
inside Ruby itself. Some use remain in a couple default gems.

Make the entire series of message lines a multiline string so that the
`Warning.warn` hook can receive them in a single call.