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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271396 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f