Add DNSSEC to SkyDNS

This adds DNSSEC. Functionality works. But it still need add more testing to check if the spec. is fully followed. If DNSSEC is not enabled the new code isn't used, so this should not trigger any unwanted behaviour in the non-DNSSEC case.

I'm putting this PR out now, because its already large. Future tweaks and more tests will come in subsequent PRs.