Input: export LEDs as class devices in sysfs

This change creates a new input handler called "leds" that exports LEDs on input
devices as standard LED class devices in sysfs and allows controlling their
state via sysfs or via any of the standard LED triggers. This allows to
re-purpose and reassign LDEs on the keyboards to represent states other
than the standard keyboard states (CapsLock, NumLock, etc).

The old API of controlling input LEDs by writing into /dev/input/eventX
devices is still present and will take precedence over accessing via LEDs
subsystem (i.e. it may override state set by a trigger). If input device is
"grabbed" then requests coming through LED subsystem will be ignored.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>