zebra: implement recursive MPLS labels

zebra: implement recursive MPLS labels

When a BGP-labeled route is resolved into an LDP-labeled IGP route,
zebra would install it with no labels in the kernel. This patch implements
recursive MPLS labels, i.e. make zebra install all labels from the route's
nexthop chain (the labels from the top-level nexthop being installed in
the top of the MPLS label stack). Multiple recursion levels are supported.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>