Add never-blank-display mode

Debugging feature that allows blocking of display state transitions
so that the display is never blanked.

Control via mcetool --set-never-blank=enabled/disabled.

The setting persists over mce restarts and device reboots.

[mce] Support for never-blank-display mode added
Merge pull request #58 from spiiroin/neverblank

Add never-blank-display mode