Ipv6 Is Not A New Thing

Most is said in the commits.
The basic idea is to reuse the (already provided) structures and streamline in the xfr-handler and server/zone structures in preparation for the zone events.

Conflicts:
	src/knot/server/xfr-handler.c
	src/knot/server/zones.c
	src/knot/server/zones.h

Forwarding Fix

Correct message id reinstating, appending of TSIG RR when forwarding query/response.

Zone Loading Failed Diff + test cases

See the commit messages, should be clear.
Also, the `failed to sign the zone:` message appears also when ixfr-from-differences fails, because it's together in one function (so it prints out even if I don't have DNSSEC signing enabled).
I'm not sure how to handle this properly, maybe have differences and zone signing in a separate functions and print the reason from within.

covered:
- data change, serial increase
- data change, serial doesn't change
- data change, serial < initial serial

zone

fails

- the zone_master API is actually useful, as it allows rotation for
  multi-master later on

throughout

- it is as good as the sockaddr_t previously, but standard
- subnets are only needed for ACLs
- common way to print addresses (even AF_UNIX)

Utils Edns

fixes #211

Conflicts:
	src/knot/server/zones.c

Zonefile loading fixes

Most is said in the commit.
I would have refactored zone-load more (especially the custom rrset lists could be standard ptrnode_t list from list.h) and a few other things,
also it should be named something like `zonefile.h` as we already have a `zone-load.h` file which is for zone loading in general and makes it confusing.
But I didnt, as @jkadlec made a lot of changes in the rrset->rrsigs and I suspect this would result in nasty collisions (like the rrset lists code).
So... this should be done after his branch is merge-able.

NSEC/NSEC3: Fixed wildcard expansion proofs and previous node lookup + tests

Most of it is written in the commit comments, it's still not pretty but at least it works for now.