Detect Power Button Devices for all supported systems

[Vincinator]

Implement a method to get a list of power button devices, so that events from all power buttons of the system can be handled by auraed.

• multiple power buttons are possible (depends on hardware)
• detect the reboot button and handle it with a reboot instead
• linux specific:
  • devices are listed in /proc/bus/input/devices
  • The integer X in /dev/input/eventX will vary. Thus, another reason why the detection is required.

Note a linux specific listener implementation: https://github.com/aurae-runtime/auraed/pull/31. BSD, OSX or other may handle input devices differently (I don't know yet).