Add additional checks for IPV6_PKTINFO handling (RFC 3542):

* Return ENETDOWN when interface specified by ipi6_ifindex is not
  enabled for IPv6 use.
* Return EADDRNOTAVAIL when ipi6_ifindex specifies an interface, but the
  address ipi6_addr is not available for use on that interface.
* Return EINVAL when ipi6_addr is multicast address.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC