dns: iterate over all configured nameservers

Previously if there were three nameservers configured and third one was
reachable via IPv6 we didn't sent query to it, since NSCOUNT6 was equal to
one. Clearly it was the original intention to sent query to all IPv6
nameservers.

Reported-by: Benedikt Gollatz <benedikt@gollatz.net>
Merge pull request #32 from msekletar/master

dns: iterate over all configured nameservers