posix_spawn(): Do not fail when trying to close an fd that is not open.

As noted in Austin Group issue #370 (an interpretation has been issued),
failing posix_spawn() because an fd specified with
posix_spawn_file_actions_addclose() is not open is unnecessarily harsh, and
there are existing implementations that do not fail posix_spawn() for this
reason.

Reviewed by: ed
MFC after: 10 days