core: increase the auxillary group limit to 65536

Make the allocation of groups dynamic and increase the limit
to 65536.

Change-Id: I702364ff460e3a982e44ccbcb3e337cac9c2df51
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5172
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Tested-by: Gluster Build System <jenkins@build.gluster.com>