posix_spawn: add chdir-related non-portable actions

Namely posix_spawn_file_actions_addchdir_np and
posix_spawn_file_actions_addfchdir_np.

Reviewed by: kevans, ngie (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33143