Allow direct access to the dirfd of a directory handle

Allow direct access to the dirfd of a directory handle

Serialization functions need access to the dirfd of a directory handle
in order to pass the fd and re-build the directory handle in
another process.

This function should not be used for other purposes. It is
also only available in the 'enable-dirfd' configuration, which
is assumed for the session daemon.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>