* use M_ZERO flag with malloc instead of explicit zeroing.
* remove MULTI_SCOPE ifdef.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Add const qualifier to in6_addrhash() function.
Add in6ifa_ifwithaddr() function. It is similar to ifa_ifwithaddr,
but does fast lookup in the hash of inet6 addresses.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC