Input: add KEY_RFKILL

Most laptops have keys that are intended to toggle all device state, not
just wifi. These are currently generally mapped to KEY_WLAN. As a result,
rfkill will only kill or enable wifi in response to the key press. This
confuses users and can make it difficult for them to enable bluetooth
and wwan devices.

This patch adds a new keycode, KEY_RFKILL. It indicates that the system
should toggle the state of all rfkillable devices.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>