systemd: handle exited kresd instances
When a kresd instance exits (could be a crash or whatever), it does not get auto-restarted by systemd like in the manager-less case. What's worse, it blocks manager itself from reloading or even ^C-quitting, as it expects the service to be running.
Tested with systemd-session controller, assuming OS-level systemd will behave similarly. This seems important to improve for production-level reliability.