Disable some more unsafe things in (low level) console mode:

Disable some more unsafe things in (low level) console mode:
- never call up to the tty layer to restart output for keyboard input in
  console mode.  This was already disallowed in kdb mode.  Other cases
  are rarely reached.
- disable the reboot, halt and powerdown keys in console mode.  The suspend,
  standby and panic keys are still allowed, and aren't even conditonal
  on excessive configuration options.  Some of these actions are still
  available in ddb mode as ddb commands which are equally unsafe.  Some
  are useful at input prompts and should be restored when the locking is
  fixed.
- disallow bells in kdb mode (should be in console mode, but the flag for
  that is not available).  Visual bell gives very alarming behaviour by
  trying to use callouts which don't work in kdb mode.  Audio bell uses
  timeouts and hardware resources with mutexes that can deadlock in
  reasonable use of ddb.

Screen switches in kdb mode are not very safe, but they are important
functionality and there is a lot of code to make them sort of work.