PowerDNS 4.x support

The main difference is that pdnssec is now called pdnsutil. Furthermore the DNSsec output is slightly different. Also, secure-zone now creates a CSK but we need the KSK/ZSK, so that command was changed to explicitly use add-zone-key instead of the 'shortcut' secure-zone command.

In any case, we should detect the correct version of PowerDNS by looking for either pdnssec or pdnsutil. The attached pr !865 (merged) should take care of this, while maintaining PowerDNS 3.x compatibility