The backlight module only enumerated and monitored the udev "backlight"
subsystem, so keyboard-backlight LEDs in the "leds" class (e.g.
white:kbd_backlight, platform::kbd_backlight) were never discovered and
the module fell back to the default when pointed at one.
Enumerate and monitor the "leds" subsystem in addition to "backlight".
Those LEDs expose the same brightness/max_brightness attributes, so the
read path is unchanged. Each device now records its subsystem so the
login1 SetBrightness call targets the correct one. Automatic device
selection still prefers a "backlight" device and only falls back to a
"leds" device when named explicitly or when no screen backlight exists.
Fixes#2848.
Alex