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.