Various file comparison improvements

Various file comparison improvements

- Rename unchanged_file() to quick_check_ok().
- Enhance quick_check_ok() to work with non-regular files.
- Add a get_file_type() function to the generator.
- Use the new functions in the generator code to make the logic simpler.
- Fix a bug where the `--alt-dest` functions were not checking if a
  special file fully matched the non-permission mode bits before
  deciding if we have found an alt-dest match.
- Enhance the `--info=skip --ignore-existing` output to include extra
  info on if the existing file differs in type or passes the standard
  quick-check logic.
- Add `--info=skip2` that authorizes rsync to perform a slow checksum
  "quick check" when ignoring existing files. This provides the uptodate
  and differs info even if we need to checksum a file to get it.