Conflicts:
	src/knot/nameserver/process_query.c

Libknot: prefixes + visibility

- All public symbols of libknot are prefixed with `knot_` or `KNOT_`.
- Private symbols are not prefixed.
- All public symbols have visibility defined (macro `_public_`).
- Symbol exporting is handled by `gl_VISIBILITY()` in `configure.ac`.

Tests pass, except for two errors (see email) that previously occurred also in `master` branch. Please, have a look at the `events/ddns` test, it may be an issue. (But if it is, it's present in master already.)

Fixes #200

Follow-up: #305, #301

See merge request !300

Conflicts:
	src/knot/nameserver/axfr.c
	src/knot/nameserver/internet.c
	src/knot/nameserver/ixfr.c

* 1.6:
  bump version to 1.6.0
  knotc: fix help message formatting
  tests-extra: zone expiration by broken AXFR
  review: mem_trim() after XFR when old zone is discarded
  schedule retry after failed AXFR
  expire zone if SOA is accepted, but AXFR is not
  events: add zone_events_is_scheduled() API
  refresh: Warning instead of Info
  refresh: Improved log message in case of older IXFR
  tests-extra: Improved/fixed test for forced refresh
  test-extra: fix Knot.ctl()
  tests-extra: refresh -f test
  refresh: Removed SOA check from AXFR processing
  tests-extra: add Knot.ctl() to allow arbitrary control operations

expire zone when XFR is failing

fixes #304

See merge request !297

Forced refresh fix

Fixes #303

See merge request !298

- Serials are checked during SOA response processing. Incoming
  AXFR should be accepted even if the serial is older. (In order
  to allow forced refersh.)

Conflicts:
	src/common/namedb/namedb.h
	src/common/namedb/namedb_trie.c
	src/libknot/descriptor.c
	src/libknot/packet/rrset-wire.c
	src/libknot/processing/requestor.c
	src/libknot/rrtype/aaaa.h

Resolver compat

See merge request !291

Extended RCODE is one 12-bit number. Until now it was not only
handled separately for old and new RCODEs, but it was also
interpreted wrong.

Unified RCODE processing - it's represented by only one number and
writing into the packet takes place at the end of query processing.

refs #300

Conflicts:
	src/Makefile.am
	src/knot/conf/conf.h
	src/knot/server/server.h

fixes: persistent timers

See merge request !294