bgpd: nexthop tracking with labels for vrf-vpn leaking

bgpd: nexthop tracking with labels for vrf-vpn leaking

Routes that have labels must be sent via a nexthop that also has labels.
This change notes whether any path in a nexthop update from zebra contains
labels. If so, then the nexthop is valid for routes that have labels.

If a nexthop update has no labeled paths, then any labeled routes
referencing the nexthop are marked not valid.

Add a route flag BGP_INFO_ANNC_NH_SELF that means "advertise myself
as nexthop when announcing" so that we can track our notion of the
nexthop without revealing it to peers.

Signed-off-by: G. Paul Ziemba <paulz@labn.net>