nfs-utils: switch gssd to use standard function for getting an RPC client

We already have a common function for setting up an RPC client. That
function uses the tirpc API when tirpc is enabled and is also already
IPv6 enabled. Switch gssd to use it.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>