routing: extend nhop(9) kpi

routing: extend nhop(9) kpi

* add nhop_get_unlinked() used to prepare referenced but not
 linked nexthop, that can later be used as a clone source.
* add nhop_check_gateway() to check for allowed address family
  combinations between the rib family and neighbor family (useful
  for 4o6 or direct routes)
* add nhop_set_upper_family() to allow copying IPv6 nexthops to
 IPv4 rib.
* add rt_get_rnd() wrapper, returning both nexthop/group and its
 weight attached to the rtentry.
* Add CHT_SLIST_FOREACH_SAFE(), allowing to delete items during
  iteration.

MFC after:	2 weeks