Add support for multicast to the bridge and allow inet6 addresses to be
assigned to the interface.

IPv6 auto-configuration is disabled. An IPv6 link-local address has a
link-local scope within one link, the spec is unclear for the bridge case and
it may cause scope violation.

An address can be assigned in the usual way;

ifconfig bridge0 inet6 xxxx:...

Tested by: bmah
Reviewed by: ume (netinet6)
Approved by: mlaier (mentor)
MFC after: 1 week