SUNRPC: Add a generic RPC credential

Add an rpc credential that is not tied to any particular auth mechanism,
but that can be cached by NFS, and later used to look up a cred for
whichever auth mechanism that turns out to be valid when the RPC call is
being made.

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