core: fix confusing logging of instantaneous jobs

For instantaneous jobs (e.g. starting of targets, sockets, slices, or
Type=simple services) the log shows the job completion
before starting:

        systemd[1]: Created slice -.slice.
        systemd[1]: Starting -.slice.
        systemd[1]: Created slice System Slice.
        systemd[1]: Starting System Slice.
        systemd[1]: Listening on Journal Audit Socket.
        systemd[1]: Starting Journal Audit Socket.
        systemd[1]: Reached target Timers.
        systemd[1]: Starting Timers.
        ...

The reason is that the job completes before the ->start() method returns
and only then does unit_start() print the "Starting ..." message.
The same thing happens when stopping units.

Rather than fixing the order of the messages, let's just not emit the
Starting/Stopping message at all when the job completes instantaneously.
The job completion message is sufficient in this case.

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