wadindex.ini: keep track of file updates in file_events

- Reworded notes for 'wads' schema block
- Reworded description for 'files' schema block
wadindex.ini: rename 'files' schema block to 'zipfiles'

- Updated field 'file_keysum' to 'zip_keysum' in 'wads' schema block
wadindex.ini: updated checksums
wadindex: updated/reformatted log output