core/manager: print status messages about running jobs

core/manager: print status messages about running jobs

Sometimes the boot gets stuck until a timeout hits. The usual timeouts
are on the order of minutes, so users may lose patience.

Print animated status messages telling the names of units with running
jobs to make it easy to see what systemd is waiting for.

The animation looks cooler with a shorter interval, but 1 s is OK and
should not be too hard on slow serial console users.