net: add ETHER_IS_IPV6_MULTICAST

This can be used to detect if an ethernet address is specifically an
IPv6 multicast address, defined in accordance to RFC 2464.

ETHER_IS_MULTICAST is still preferred in the general case.

Reviewed by: ae
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26611