Rewrite turris.omnialeds module to be more flexible

Exposes LED settings as RFC42-style "settings" attribute, so in most cases writing commands manually is not needed. Also improves the speed at which LED settings are set by running them in parallel, and turns off the LEDs when stopping the service (i.e. on shutdown/reboot).

LED settings are now under turris.omnialeds.led.NAME where NAME is the name of the LED as exposed in /sys/class/leds, without the rgb: prefix.

Example:

{
  turris.omnialeds.led = {
    power.settings.invert = true; # Make heartbeat blink off instead of on
    indicator-2.trigger = "disk-activity";
  };
}