nhops: split nh_family into nh_upper_family and nh_neigh_family.

[melifaro]

Summary

With IPv4 over IPv6 nexthops and IP->MPLS support, there is a need
to distingush "upper" e.g. traffic family and "neighbor" e.g. LLE/gateway
address family. Store them explicitly in the private part of the nexthop data.

While here, store nhop fibnum in nhop_prip datastructure to make it self-contained.

MFC after: 2 weeks

[melifaro]

add nhop_get_fibnum().