gh-113117: Support posix_spawn in subprocess.Popen with close_fds=True (#113118)

Add support for `os.POSIX_SPAWN_CLOSEFROM` and
`posix_spawn_file_actions_addclosefrom_np` and have the `subprocess` module use
them when available.  This means `posix_spawn` can now be used in the default
`close_fds=True` situation on many platforms.

Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org>