zebra: add new flag to detect nexthop label updates

zebra: add new flag to detect nexthop label updates

With the introduction of the pseudowire manager, the NHT tracking code
needs to detect label updates as well.

Create a specific nexthop flag for that. We can't reuse the
RIB_ENTRY_NEXTHOPS_CHANGED flag for this porpose because this flag is
always cleared and reevaluated in rib_process(), setting it outside that
function is a nop.

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