posix_spawn_file_actions_adddup2(3) on FreeBSD

The text 85c032bd0df1, as FreeBSD 14.0-RELEASE, FreeBSD 13.5-RELEASE, FreeBSD 13.4-RELEASE, FreeBSD 13.3-RELEASE shipped it.

add open, dup2, close, closefrom, or chdir/fchdir actions to spawn file actions object