mount.nfs: proto=netid forces address family when resolving server names

Using the netid settings, determine the correct address family to use
for NFS and MNT server name resolution.  Use this family when
resolving the server name for the addr= and mountaddr= options.

This patch assumes the kernel can recognize a netid, instead of a
protocol name, as the value of the proto= options.

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