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