evdev: disable evdev if it is invoked from KDB or panic context

This allow to prevent deadlock on entering KDB if one of evdev locks is
already taken by userspace process.

Also this change discards all but LED console events produced by KDB as
unrelated to userspace.

Tested by: dumbbell (as part of D15070)
Objected by: bde (as 'KDB lock an already locked mutex' problem solution)
MFC after: 1 month