manager: rework sending of STATUS=

We would send READY=1,STATUS="Startup finished in …" once after finishing
boot. This changes the message to just "Ready.". The time used to reach
readiness is not part of the ongoing status — it's just a bit of debug
information that it useful in some scenarious, but completely uninteresting
most of the time. Also, when we start sending status about other things in
subsequent patches, we can't really go back to showing "Startup finished in …"
later on. So let's just show "Ready." whenever we're in the steady state.

In manager_check_finished(), more steps are skipped if MANAGER_IS_FINISHED().
Those steps are idempotent, but no need to waste cycles trying to do them
more than once.

We'll now also check whether to send the status message whenever the job queue
runs empty. If we already sent the exact same message already, we'll not send
again.