rfkill: Add rfkill-any LED trigger

Add a new "global" (i.e. not per-rfkill device) LED trigger, rfkill-any,
which may be useful on laptops with a single "radio LED" and multiple
radio transmitters.  The trigger is meant to turn a LED on whenever
there is at least one radio transmitter active and turn it off
otherwise.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>