Various backend improvements.

- Rewrite windows loader to use standard Toolhelp32 snapshots instead of
  hardcoding internal Windows structures with GetPebAddress.

- Refactor file-finding code to find_files_list in utils.h.

- Add test for existence of game_data.ini.

- Rewrite read_* functions in favor of passing void * pointers directly
  to read_raw.
Merge branch 'windows' into DF2014