- knot_rrset_copy now returns pointer

- knot_rrset_create_rr removed, only way to insert RR into rrset now is
  when you already have RDATA, so that we never create RRSet that is not
sorted.
- rr.h doxygenated, functions moved around a bit.

 - merging can now be done directly using knot_rrs_t

 - tests are passing, however some work is still pending, namely storage
   of additionals for freeing.
 - removed some obsolete node API functions

- if adding apex NS, adjust counter

- Do not merge incoming RRs
- Duplicated add/remove checks improved in ddns
- Remove stuff directly from RRSet, do not return

- removing and adding already existing record
- leak solved, but need to move old a new data lists from
  zone_contents_t

- sem checks missing, more extensive tests missing

- most checks can be done without allocating anything

- Lost my temper with the direct changes, rewritten DDNS to only
  changeset creation, application will be the same as usual.
- Will not work now

updates.

- DDNS and changeset application will not work now, but it wouldn't work
  before either

- some cleanup and fixes as well, namely simplified RR creation during
  zone load (no malloc)

- RRSet copies removed, packet now stores rrsets directly (structure is
  copied, but copy is shallow)

- use rrset initialization macro instead, or create rrset explicitely

- kept RRSet copies somewhere (node has to create them now)
- replaced knot_rrset_t with knot_rrs_t elsewhere
- semantic checks simplified

- Will leak
- Untested

No need to be picky about this, TSIG rcode is 16bit so we might
as well use 16bit number to keep things the same.

Fixes #218

The config.h is now implicitly included using -include option of the
compiler. After this change, config.h need not be placed at the first
position in all source files.

Was ignoring RRSIGs related to a RRSet that was completely removed.
This was a relict of old code when RRSIGs were connected to their
RRSets.

Also simplified the function
(and function sign_rr_should_be_signed()) and did some minor
refactoring and polishing.

- Most changes were in ddns.c: Empty checks used rr_count == 0

- Removed rrset->rdata_indices, RDATA lengths are now stored inline together with RR's TTL
- Added function to get RR's TTL - TTL of the whole RRSet can still be obtained, but it will default to the first RR's TTL or 0
- Renamed few RRSet functions, removed a few too - the file still needs a cleanup and function reordering
- Special handling for RRSIGs during AXFR-out no longer needed (yay)