kbdmux(4) keyboard multiplexer integration

o Add two new ioctl's KBADDKBD and KBRELKBD. These are used to add and remove

keyboard to (and from) kbdmux(4) keyboard multiplexer;

o Introduce new kbd_find_keyboard2() function. It does exactly the same job

as kbd_find_keyboard() function except it allows to specify starting index.
This function can be used to iterate over keyboards array;

o Re-implement kbd_find_keyboard() as call to kbd_find_keyboard2() with starting

index of zero;

o Make sure syscons(4) passed KBADDKBD and KBRELKBD ioctl's onto currently

active keyboard.

These changes should not have any visible effect.

MFC after: 1 week