mount.nfs: support netids in nfs_options2pmap()

When parsing mount options in nfs_options2pmap(), treat the value of
proto= (and mountproto=) as a netid by looking it up in local
netconfig and protocol databases to convert it to a protocol number.
If TI-RPC is not available, the traditional behavior is preserved.

The meaning of the "udp" and "tcp" mount options is not affected by
this change.

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