rpc: For AF_UNIX sockets, do not set keepalive option.

Change-Id: I65df52e89fe7783ff00c2b7a49be571d15e8f0d0
BUG: 920009
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Reviewed-on: http://review.gluster.org/4684
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>