job: fix loss of ordering with restart jobs

Suppose that foo.service/start is a job waiting on other job bar.service/start
to finish. And then foo.service/restart is enqueued (not using
--ignore-dependencies).

Currently this makes foo.service start immediately, forgetting about the
ordering to bar.service.

The runnability check for JOB_RESTART jobs looks only at dependencies for
stopping. That's actually correct, because restart jobs should be treated the
same as stop jobs at first. The bug is that job_run_and_invalidate() does not
treat them exactly the same as stop jobs. unit_start() gets called without
checking for the runnability of the converted JOB_START job.

The fix is to simplify the switch in job_run_and_invalidate(). Handle
JOB_RESTART identically to JOB_STOP.
Also simplify the handling of JOB_TRY_RESTART - just convert it to JOB_RESTART
if the unit is active and let it fall through to the JOB_RESTART case.
Similarly for JOB_RELOAD_OR_START - have a fall through to JOB_START.

In job_finish_and_invalidate() it's not necessary to check for JOB_TRY_RESTART
with JOB_DONE, because JOB_TRY_RESTART jobs will have been converted to
JOB_RESTART already.

Speeding up the restart of services in "auto-restart" state still works as
before.

Improves: https://bugzilla.redhat.com/show_bug.cgi?id=753586
but it's still not perfect. With this fix the try-restart action will wait for
the restart to complete in the right order, but the optimal behaviour would be
to finish quickly (without disturbing the start job).