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 (closed) Following: #301 (closed)