Keyboard notifier
  
  One can use register_keyboard_notifier to get called back on keyboard
  events (see kbd_keycode() function for details).  The passed structure is
  keyboard_notifier_param:
  
  - 'vc' always provide the VC for which the keyboard event applies;
  - 'down' is 1 for a key press event, 0 for a key release;
  - 'shift' is the current modifier state, mask bit indexes are KG_*;
  - 'value' depends on the type of event.
  
  - KBD_KEYCODE events are always sent before other events, value is the keycode.
  - KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
    value is the keycode.
  - KBD_UNICODE events are sent if the keycode -> keysym translation produced a
    unicode character. value is the unicode value.
  - KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
    non-unicode character. value is the keysym.
  - KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
    That permits one to inspect the resulting LEDs for instance.
  
  For each kind of event but the last, the callback may return NOTIFY_STOP in
  order to "eat" the event: the notify loop is stopped and the keyboard event is
  dropped.
  
  In a rough C snippet, we have:
  
  kbd_keycode(keycode) {
  	...
  	params.value = keycode;
  	if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
  	    || !bound) {
  		notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
  		return;
  	}
  
  	if (unicode) {
  		param.value = unicode;
  		if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
  			return;
  		emit unicode;
  		return;
  	}
  
  	params.value = keysym;
  	if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
  		return;
  	apply keysym;
  	notifier_call_chain(KBD_POST_KEYSYM,&params);
  }
  
  NOTE: This notifier is usually called from interrupt context.