Modified answering algorithm + fixed DNSKEY adding

- Infinite loop replaced by several labels and gotos.
- This fixed problems with CNAME resolving related to closest
  enclosers saved in dname.node.
- Fixed DNSKEY adding - was not checking if the RRSet is there.

TODO: Try to refactor the answering code
TODO: Simplify DNSSEC records adding by marking the zone as signed
      / unsigned.

fixes #640