Check that a zone is served by IPv6 servers if it has AAAA records

One thing that is often forgotten when turning on an IPv6 service is to ensure that the zone holding the AAAA records for that service is also served over IPv6. This can be relatively easy be checked for by named-checkzone by looking for AAAA records in the zone contents, including glue AAAA records, and then checking that there are AAAA records published for some of the nameservers if any are found (in zone or elsewhere). This can also sometimes be determined by named without needing to look beyond the zone's contents, but cannot be guaranteed.