Add experiment to use runit force-stop, force-restart

After this is merged, people using Runit can opt-in to "force restarts" and "force stops". This means that after a timeout of 7s, Runit will send SIGKILL when stopping or restarting.

To participate in the experiment, set GDK_RUNIT_FORCE_STOP=1 in your environment. After that, gdk stop and gdk restart should use SIGKILL when something (e.g. Sidekiq) takes too long to stop.

Hopefully this can fix #620 (closed).