Changes to HANDLE_AUDIO and HANDLE_TILESETS

- Add the output_path option
- Refactor away the MOVEs.
Update documentation of HANDLE_TILESETS to reflect recent changes
Add yet more constants for GET_OFFSET_ARRAY and GET_OFFSET_ARRAY2
Minor refactoring to fj_are_structure

- Use standard constants for GET_OFFSET_ARRAY and
  GET_OFFSET_ARRAY2 when possible
- Notably, this fixes an incorrect offset for AREA V9.1
Fix white space in biff.ml
Fix a bug in the bounds checking when unbiffing tilesets
Document [READ|GET]_STRREF_[F|S|FS] and the corresponding actions.