Each BSD flavor has slightly different way to handle CPU counting,
CPU set management and affinity setting.

refs #1882

Partial core overlap was implemented to prevent stalls and
better utilization of free cores by the scheduler. Not portable.

- No error returned if the incoming packet has less than 2 bytes.
- Other cases should be handled properly.

refs #1867 @30min

- When returning general error response (i.e. without the
  Question section), the OPCODE and RD bit were not copied.

TODO: handle shorter query wire formats in some cases.

refs #1867 @2h

refs #1828 @2h

Notation of 'ip:port' was indistinguishable for IPv6.

No problem at all to use standard malloc().
May be replace with custom allocator later, but it is not an issue.
Buffers for UDP answering are per-thread only.

refs #1665

refs #1598

refs #1580, #1583, #1584, #1585, #1586, #1587, #1441, #964,  #278, #1542, #1348

Required for saving transferred zones.

refs #1556

refs #1540

TODO:
* OpenBSD posix_memalign() is broken, Slab allocator is disabled.

refs #1551

refs #1353

- Address checking in normal queries.
- Fixed some typos.

refs #1496 @1h

- 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

refs #1442, #1440

fixes #1342

refs #1321 @20m

* AXFR/UDP query is responded as FORMERR (according to BIND and RFC1034)
* IXFR/UDP query is responded with SOA (according to RFC1995)
* Experimental support for (A|I)XFR/UDP is implemented, but disabled.
** I did not encounter server supporting full IXFR/UDP nor client tool to test it.
** AXFR/UDP is still in state of draft and not supported, so disabled as well.

refs #984

refs #1322

refs #1322

Commit refs #1253.

To enable again, see SLAVE DISABLED in code.

Commit fixes #837.