text-based mount command: support AF_INET6 in rewrite_mount_options()

Now that we have an AF_INET6-capable probe_bothports(), we can support
AF_INET6 when rewriting text-based NFS mount options.  This should be
adequate to support NFS transport protocol and version negotiation with
AF_INET6 NFS servers.

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