land b_groups onto HEAD:

 - add a flag in lustre_msg which indicate message type
 - add a security descriptor into most of messages send to MDSs, which
   include the uid, supplementary groups array etc.
 - add group hash
 - MDS retrive local groups database through upcall, and set/merge with
   which sent from client based on policies (here not totally finished)
 - move root_squash test into separate sanity-sec.sh