Fix automatic detection of powerkey input devices

Instead of relying on driver names, the code now detects
devices that can send KEY_POWER events.

In case there are several devices that can emit power key
events, all are tracked. However no attempt is made to try
to separate simultaneous powerkey presses from several
sources.
Normalized whitespace and fixed a typo
Merge pull request #2 from spiiroin/powerkey_fix

Fix automatic detection of powerkey input devices