rpc: bring in root-squashing behavior in rpc

* requests coming in as root are converted to nfsnobody

* with open-behind some acl checks wont happen and nfsnobody
  can read the file "whose owner is root and other users do not
  have permission to read the file". This is becasue open-behind
  does not send the open to the brick and sends success to the
  application, thus the acl related tests on the file wont happen
  which would have prevented the file from being opened.

Change-Id: I73afbfd904f0beb3a2ebe807b938ac2fecd4976b
BUG: 887145
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4516
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>