rec: Allow the use of a 'self-resolving' NS if cached A/AAAA exists

rec: Allow the use of a 'self-resolving' NS if cached A/AAAA exists

We just have to take care not to try to contact that NS to learn
its own IP addresses, because that does not make sense.
Before this, we could skip a perfectly valid NS for which we had
retrieved the A and/or AAAA entries, for example via a glue.
Also get rid of a flawed calculation based on whether IPv6 was
enabled whereas we were only dealing with NS at this point.