svc: Make svc_sock the tcp/udp transport

Make TCP and UDP svc_sock transports, and register them
with the svc transport core.

A transport type (svc_sock) has an svc_xprt as its first member,
and calls svc_xprt_init to initialize this field.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>