No reasoning behind this change, except that it seems to work more
reliably. On my machine getfqdn() returns 'localhost.localdomain',
while Knot detects correct system canonical name. On distel, both
functions return the same result.

fixes #226

Conflicts:
	src/knot/zone/semantic-check.c

close #216

Conflicts:
	src/knot/zone/node.c
	src/knot/zone/node.h
	src/knot/zone/zone-contents.c
	src/knot/zone/zone-contents.h

Efficient node data storage, libknot refactoring

Changes:

- new file rr.h:
  - contains functions and structures that work with RRs or RR arrays.
  - this enables us to have RRSet-like functionality inside `node_t`

- node no longer stores `knot_rrset_t`, but only structure containing type, data and additional nodes.

- RRSet API cleaned up a lot:
 - RRSet is now an entrypoint into zone structure
 - It is no longer possible to create RRSet that is not sorted or one that contains duplicates
 - Obsolete getter functions removed

- Node and zone contents API cleaned up

- DDNS rewritten to changeset creation only.
 - Still far from perfect, but this is the best I can do without new zone API
 - Over-complicated prereq check rewritten too

- Changeset application greatly simplified, no need to explicitly handle changes anymore
 - Application now preserves the changeset structure, everything is copied
  - We could probably simplify the code a lot because of this at many places

Conflicts:
	src/knot/dnssec/zone-sign.c
	src/knot/updates/ddns.c
	src/knot/updates/xfr-in.c
	src/knot/zone/node.c
	src/knot/zone/node.h
	src/knot/zone/semantic-check.c
	src/knot/zone/zone-contents.c
	src/knot/zone/zone-contents.h
	src/knot/zone/zone-create.c
	src/libknot/rrset.c
	tests-extra/tests/ddns/basic/test.py

Note: ddns/basic fails now, but I don't think it's caused by my merge
jutsu.

- ns_put_rr function now accepts consts rrsets only
- fixed wrong TTL set in SOA copy
- removed a function move_rrset, to be replaced with RRSet function
  later.

Ddns When Zone Is Reloaded

See the commit message.