rpc-transport: Moved unix socket options function to rpc-transport

This change removes the asymmetry in the 'layer' (read rpc, transport
etc) in which transport options were being filled for inet and unix
sockets.

Change-Id: Iaa080691fd5e4c3baedffa97e9c3f16642c1fc12
BUG: 955919
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4850
Reviewed-by: Raghavendra G <raghavendra@gluster.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>