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