Improved --dirs/--no-dirs/--list-only option handling:

- Moved setting of list_only and xfer_dirs from main.c to options.c.
- Fixed the ability of the user to force --no-dirs.
- Added the --old-dirs/--old-d option to make it easier to interact
  in list-only mode with an older rsync.
- Suggest the use of --old-d instead of "-r --exclude='/*/*'".