Add posix_spawn_file_actions_add{f}chdir_np

Add posix_spawn_file_actions_add{f}chdir_np

These are defined as _np functions and available in glibc, musl, macOS,
FreeBSD, Solaris ≥ 11.3

They are likely to be standardized without the _np suffix as a result of
Austin Group issue 1208. if so, both names will be kept as aliases.

Introduce HAVE_CHDIR and HAVE_FCHDIR to allow building on systems not
providing these calls.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>