Bug 72680 - 'user-config.py' not found if runs with cron

Changed search order for user-config.py.
See also bug 72918.

New order:
1. -dir: argument
2. PYWIKIBOT2_DIR
3. .
4. pwb.py directory
5. 'pywikibot' folder under:
      'Application Data' or 'AppData\Roaming' (Windows) or
      '.pywikibot' directory (Unix and similar) under the user's home

Also code improvements:
- in search logic
- removed TODO: Raise exception on not handled windows version

Bug: 72680
Change-Id: Id23afcdd2fa1a17710ca786e69c6ea985b86e870
Merge "Bug 72680 - 'user-config.py' not found if runs with cron"