Ruby

Release GVL around system calls in dir.c

• 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.