Active signal mask is inherited by new threads. Therefore, control
signals used by the main thread had to be blocked at the moment, when
new threds are created.

The former mask used to block signals was empty and had no effect.

The signals are blocked during server shutdown. Therefore the signal
handler could not be called twice.

Improve structure names

See merge request !307

Discarded the libknots (common-knot) library

Most of the files were moved to `knot/common/` (ex `knot/other/`).

Two things moved to `utils/common`, because were used only there (`hex`, `strtonum`).

One file (`strlcat`) moved to `libknot/internal/`, as it was used both in server and in libknot. (Also, `strlcpy` was there already).

See merge request !306

Most of the files were moved to knot/common/ (ex knot/other/).

Two files moved to utils, because were used only there
(hex, strtonum).

One file (strlcat) moved to libknot/internal/, as it was used both
in server and in libknot. (Also, strlcpy was there already).

Moved common/ to libknot/internal/

2nd step in separating libknot into an independent library. As we agreed, I moved all files from `common/` to `libknot/internal/`.

Also some renaming was done: symbols in `libknot/internal/` should not be prefixed with `knot_` unless they are system symbol replacements. Unified the prefixing of `namedb` symbols.

Fixes #305
Following: #301

See merge request !303

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

Conflicts:
	src/libknot/internal/namedb/namedb.h
	src/libknot/internal/namedb/namedb_lmdb.c
	src/libknot/internal/namedb/namedb_trie.c

closes #300

Symbols in libknot/internal/ should not have knot_ prefix unless
they are a replacement for a system function.

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