Handle keyboard backlight toggle keys.

Pressing the XF86XK_KbdLightOnOff (kbdillumtoggle) will cycle through
the available brightness levels (it will increase brightness up to the
maximum and then go back to minimum).

If the keyboard has only two states, this behaves as a simple toggle.

The reason for cycling is that it is unlikely that a laptop will have
all three buttons for brightness up, down and toggle. If the machine
has only a toggle button and more than one brightness level, a simple
on/off (or min/max) toggle will not allow to use the intermediate
levels.

Closes: #222