Consistently use if_link.

Reviewed by:	ae, melifaro
Remove link-local multicast routes remnants from in6_purgeaddr.
Also merge in6_purgeaddr_mc with in6_purgeaddr.

Sponsored by:	Yandex LLC
* Make nd6_dad_duplicated() constant.
* Simplify refcounting by using nd6_dad_add() / nd6_dad_del().

Reviewed by:	ae
MFC after:	2 weeks
Sponsored by:	Yandex LLC
Add sa6_checkzone_ifp() function. It checks correctness of struct
sockaddr_in6, usually obtained from the user level through ioctl.
It initializes sin6_scope_id using given interface.

Sponsored by:	Yandex LLC