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

Conflicts:
	Knot.files
	src/libknot/internal/hhash.c

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