+ Some small fixes in nameserver.

refs #1128 @30m

refs #1128 @30m

refs #1128 @10m

refs #1128 @30m

- Main function + distinguishing various types of prereqs.

refs #1128 @1h

refs #937 @15m

- Added function ns_process_update() that uses (not yet implemented)
  functions from ddns.
- Extracted error creating into separate private function
  ns_error_response_full().
- Separated response preparing into private function
  ns_prepare_response().
- Modified ns_answer_normal() accordingly (TODO: test).

refs #937 @1h

refs #937 @5m

refs #937 @15m

refs #929

refs #979

refs #1103

Adding empty NSID data is completely legal. It is however a matter
of discussion if the function should add the EDNS OPTION or not.

The arguments supported are OK from the viewpoint of the function.

Conflicts:
	src/tests/libknot/libknot/rdata_tests.c

The parse_next_rr_answer() function does not require the pointer
to be NULL.

Closes #1081

+ rdata free test marked TODO.

fixes #1101 @15m

@4h

- Fixed error functions.