quagga: add MPLS/LDP support
zebra: no need to check if route's distance is infinite if it is selected

In the Zebra's route selection process, routes with infinite distance are not
eligible to be selected.
zebra: remove unused nexthop attribute
zebra: disconsider the default route when evaluating recursive nexthops

This is the behavior adopted by most implementations.
zebra: disconsider removed routes in zebra_route_node_active()