IPv6 address configured while interface is down bypasses Duplicate Address Detection entirely

Summary

in6_update_ifa(:1074-1077) only sets IN6_IFF_TENTATIVE when in6if_do_dad true (requires IFF_UP+IFF_RUNNING, :2446). Address added while down: never tentative. in6_if_up_dispatch(:2406) only DADs tentative addresses. No DAD ever run -> duplicate address on-link. XXX acknowledged by code.