NFS/SUNRPC: support transport protocol naming

To prepare for including non-sockets-based RPC transports, select
RPC transports by an identifier (to be used in following patches).

Signed-off-by: Tom Talpey <tmt@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>