Does not implement hook_file_download the right way and hijacks path system/files

Private Download is a nice little module I have been using on several of my sites with no problem.
Until I installed it on my latest site and noticed that something was going wrong with one of my custom modules that implements hook_file_download.
I then had a look at Private Download's code and found naughty things out there!

- First, PD hijacks path system/files by adding its own access arguments entry.
- Second, PD does not implement hook_file_download the right way, as it does not check any permission, although it is the purpose of this hook (See http://api.drupal.org/api/drupal/developer--hooks--core.php/function/hoo...).
Actually, the access check wrongly added to the system/files entry should be performed inside hook_file_download.
Obviously, PD works flawlessly if it is the only module installed that implements hook_file_download. But as it breaks the way path system/files and hook_file_download are supposed to work, it will conflict with other modules that depend on this hook too.