SUNRPC: create stubs for xprtsock init and cleanup

Over time we will want to add some specific init and cleanup logic for the
xprtsock implementation.  Add stub routines for initialization and exit
processing.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>