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>
    
    Resolves: #966174