Todo:
1. add option to enable/disable "exit" menu item
2. add command line arg to set or restore backlight, freq etc. powerctrl can replace lightnpower.py within xiniitrc
3. add option to check backlight and restore.
4. Since mbmenu cost 4MB memory, try to find a way to merge mb-applet-powerctrl's menu to main menu.
5. check swapfile on cf/sd, add option to keep swapfile when swap off. This function rapid swap on.