mount/fuse: expose 'glusterfs.gfid*' virtual xattr key

currently two keys are exposed:

'glusterfs.gfid' : output is 16byte binary gfid
'glusterfs.gfid.string' : output is 36 byte canonical format of gfid

e.g.
[root@supernova glusterfs]# getfattr -n glusterfs.gfid -e hex f0
glusterfs.gfid=0x68305acb73e541719804fcf36a4857e8

[root@supernova glusterfs]# getfattr -n glusterfs.gfid.string f0
glusterfs.gfid.string="68305acb-73e5-4171-9804-fcf36a4857e8"

early consumers for this key would be geo-replication
(as it has being designed to do namespace operations on
gfid from the mount point, thereby needing the GFID for
entry operations on the slave).

Change-Id: I10b23dbd11628566ad6924334253f5d85d01a519
BUG: 847839
Original Author: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/5129
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>