zebra: implement recursive MPLS labels

zebra: implement recursive MPLS labels

When a nexthop is resolved via a label based nexthop, copy
the labels into the newly created recursive nexthop.

Please note that this does not fix the case where we
have a label based nexthop that is recursively resolved
through *another* nexthop that is also label based.
In this case we need to create a new label stack
for those routes.

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