mount.nfs: return error if proto= option specified IPv6 when IPv6 isn't supported

Right now, there's nothing that expressly forbids someone from
specifying proto=tcp6 for instance, even when nfs-utils it built without
IPv6 support. This may not work well if (for instance) they are using
NFSv3, since statd won't support IPv6. Explicitly return an error if
someone specifies an IPv6 proto= or mountproto= option and IPv6 isn't
supported.

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