Namedb iter seek

This merge request extends the namedb api for following operations:
- `clear()` which equals to deletion of all entries in the database
- `iter_seek()` which returns an iterator to given key (or first, last, leq, geq)
  - this is not supported in the trie yet, but it works for lmdb backend
The unittests are extended to cover these operations.

See merge request !302

Not supported in trie because of the API incompatibility.

Allow server to start before the interfaces are up

This adds very simple support for starting server before all interfaces are up (see [1]).  The most correct solution would be to use rtnetlink[2] and bind to interfaces as they come up, since using IP_FREEBIND or IP(V6)_BINDANY also means that wrong IP address won't be detected and reported as error.

Something like this is needed for systems with systemd as init system and dynamic interfaces.

NSD is also haunted by this (see [3]).

References:
1. http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/
2. http://man7.org/linux/man-pages/man7/rtnetlink.7.html
3. https://bugs.debian.org/765831

See merge request !295

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