init.d: correcting rbdmap LSB header / init order:

 * Require "$remote_fs" since it guarantees /usr availability
   (rbd executable is in /usr/bin/rbd)
 * Speed-up init.d rbd mapping on machines acting as MON/OSD
   by starting rbdmap after /init.d/ceph (when possible) and
   shutting down rbd before ceph.
 * Map rbd devices before starting X (helpful when /home is mounted from rbd).