Ensure 'labels' file cached is deleted.

When using os-prober with direct access, ensure the cache file named
'labels' is deleted before running the detection.
Drop Freedesktop (xdg-open) support for root.

xdg-open is not reliable as root. So pick a valid graphical editor
amongst:
- leafpad
- gedit
- geany
- kate
- 'xterm -e nano' as the last ressort.