Fix mld6query(8) and add a new -g option

The mld6query command relies on KAME behaviour which allows the
ipv6mr_multiaddr member of the request object in a IPV6_JOIN_GROUP
setsockopt() call to be INADDR6_ANY. The FreeBSD stack doesn't allow
this, so mld6query has been non-functional.

Also, add a -g option which sends a General Query (query INADDR6_ANY)

Reviewed by: sbruno, mmacy
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D15384