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

(cherry picked from commit fe05d1dd0fdf7c8d09b9aad4ff74c44b7c90db80)