Make it possible to match on path-id in bgpctl show rib outputs.

Make it possible to match on path-id in bgpctl show rib outputs.
To work properly also a neighbor needs to be selected. The assumption
here is that the peer will use the same path-id for the same peer
accross all its routes. The RFC does not require this and it is valid
to assign path-ids randomly. The path-id only matters for one specific
path but most BGP implementations seem to assign the same path-id to
multiple routes when originated from the same source.
OK benno@