Make mpe(4) work again by:

Make mpe(4) work again by:
- Change the way mpe figures out the IP of the MPLS nexthop. Instead of using
RTF_GATEWAY and so a valid (and cachable) gateway route just use the
gateway IP address of the route (rt->rt_gateway).
- Make sure the interface is up when adding a mplslabel. The inserted route
is in rtable 0 and so invisible for the link state tracker. Forcing the if_up
ensures that the added route is RTF_UP.
OK mpi@