Input: rename SW_RADIO to SW_RFKILL_ALL

The SW_RADIO code for EV_SW events has a name that is not descriptive
enough of its intended function, and could induce someone to think
KEY_RADIO is its EV_KEY counterpart, which is false.

Rename it to SW_RFKILL_ALL, and document what this event is for.  Keep
the old name around, to avoid userspace ABI breaks.

The SW_RFKILL_ALL event is meant to be used by rfkill master switches.  It
is not bound to a particular radio switch type, and usually applies to all
types.  It is semantically tied to master rfkill switches that enable or
disable every radio in a system.

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