[j-nsp] Having the same interface in multiple routing instances

[Phill Jolliffe]

As Stefan mentions one logical unit must to belong to one routing
instance. But you can play (ugly) games with rib groups and interface
routes. The newer instance import policies feature should work for non
VRF instance as well but I only implemented it with RIB groups so far.

The logical unit will always belong to the one RI and all *incoming*
packets will be compared by default against this RI-name.inet.0 table.
But it is possible to copy the interfaces routes from another instance
into your new instance. So now packets in both instances can be
forwarded *out* of the routing instance.

This is a little messy config wise but means you can now use one
interface for outbound in multiple routing instances.

If we make the config even more ugly we can use policy based routing,
(FBF), to take incoming traffic on the one interface and control which
of the routing instances we use for routing lookup as well.

Cheers

Phill