dns64: workaround for broken auths which fail on AAAA query

dns64: workaround for broken auths which fail on AAAA query

Some auths return weird stuff on AAAA query and RFC 6147 section-5.1.2
mandates workarounds for these cases.

This is only partial fix.
In some cases all servers return answers so weird that iterator refuses
to accept them and fails with "no NS" condition, which in turn does not
call consume phase and skips dns64 module.
Fixing this particular case will require changes to module logic and
such change is not suitable to minor release.

Related: #483