MFC r339823:

evdev: Use console lock as evdev lock for all supported keyboard drivers.

Now evdev part of keyboard drivers does not take any locks if corresponding
input/eventN device node is not opened by userland consumers.

Do not assert console lock inside evdev to handle the cases when keyboard
driver is called from some special single-threaded context like shutdown
thread.

MFC r339824:

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.