Find a way to restart slim no matter what

Starting with 0.7.0, we no longer have text-mode consoles if the computer was started with `boot_mute` (which is the default). This can lead to the situation that the user types `sudo service slim stop`, resulting in a gray screen with no possibility to interact further with the system (unless ssh is enabled).

Hence, we need to make slim restart when the service is stopped or killed for whatever reason.

Put `service slim start` in `/etc/ttys`, possibly wrapped into a script that stays around as long as Xorg is running?