Add option for disabling power key when proximity is covered

Similarly to double tap, also power key actions can now be
* always allowed
* never allowed
* allowed if proximity sensor is not covered

Default is "always", i.e. power key works the same way as before.

The setting can be changed via mcetool option
  -Z, --set-powerkey-action=<never|always|proximity>

The setting persists over mce / device restarts.

[mce] Add option for disabling power key when proximity is covered