This also introduces
NMPtrArray helper. The branch uses the new type in the new function
nmp_object_new_from_nl_v(). Still, the real reason why
nmp_object_new_from_nl_v() is added is not yet obvious. It will be required in a future patch, where -- when we process the first next-hop of an IPv6 route -- we need to know the entire list of next hops.