Config hostname errors

Included changes:

- log warnings properly
- issue warning when FQDN retrieval fails for system.identity and system.nsid
- cache hostname lookup

refs #306

See merge request !304

closes #300

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

Some RCODEs other than NOERROR may result in DNSSEC processing,
e.g. NXDOMAIN.

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

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