Allow Powerdown menu for devices other than M1

Just because device can't power itself off, doesn't mean system shouldn't be given opportuntity to have a clean exit from xbmc.
Add lastgood configuration go Loadsettings for guisettings

If guisettings == OK, then make a copy to .lastgood
If guisettings != OK, then rename to .bad and try to load .lastgood
if guisettings != OK, and .lastgood not exist or bad then Reset.
Make sure to delete lastgood before writing new one.