- This is due to TSIG checking which must be done outside libknot.
- New function knot_ns_prep_normal_response() for preparing the
  response structure and finding zone for answering.
- knot_ns_answer_normal() simplified to just answer using the
  prepared response structure.
- New API function zones_normal_query_answer() that calls these two
  functions and checks TSIG before calling the answering function.

refs #1496 @1h