Input: add a new EV_SW SW_RADIO event, for radio switches on laptops

Many laptops have rf-kill physical switches that are not keys, but slider
or rocker switches.  Often (like in all ThinkPads with a radio kill slider
switch), they have both a slider/rocker switch and a hot key.

Trying to kludge a real switch to act like a key is not a very smart thing
to do if you can help it, and it gets specially bad when you are going to
have both in the same machine.  So, we do the right thing and add an input
EV_SW event for radio kill switches.

The EV_SW SW_RADIO event is defined with positive logic, i.e. when the
switch is active, the radios are to be enabled.  When the switch is
inactive, the radios are to be disabled.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>