ldpd: update local labels when necessary

ldpd: update local labels when necessary

ldpd allocates null labels for directly connected routes. If a connected
route is removed (interface goes down) and an IGP learned route takes its
place in the RIB, ldpd must update the local label of the associated FEC
entry with a non-null label. The same applies for the other way around
(an interface goes up and a connected route is selected in favour of an
IGP route). Labels should be dynamic and change when necessary.

Additionally, this patch fixes the processing of route delete messages
from zebra. Route delete messages don't contain any nexthop, meaning that
whenever we receive such messages we must delete all nexthop previously
received.

Based on a patch from Bingen Eguzkitza <bingen@voltanet.io>.
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>