The answer is conf_path().
The backstory is we are making a new 'modules', 'themes', or 'files' level folder inside sites/whatever: 'config', to store configuration settings in files.
This would be the ultimate way to get a listing of all ini files in modules, and could do the same thing for 'config' -- this grabs from the currently in use sites directory (such as sites/default) as well as sites/all, profiles, and the built-in modules directory:
drupal_system_listing('\.ini$', 'modules', 'name', 0);