manager: SIGINT while shutting down should lead to immediate termination

In weird cases such as when you kill all kresd instances by hand while manager is slowly stopping them by itself, the manager can get stuck (in general it happens while waiting for systemd jobs which never finish).

Control+C in these cases does nothing. It triggers shutdown, but the shutdown waits for the stuck task. Usability would be improved quite a bit if we detected these cases and terminated forcefully.