run: propagate return code/status from the child

Fixes #13756. We were returning things that didn't make much sense:
we would always use the exit_code value as the exit code. But it sometimes
contains a exit code from the process, and sometimes the number of a signal
that was used to kill the process. We would also ignore SuccessExitStatus=
and in general whether systemd thinks the service exited successfully
(hence the issue in #13756, where systemd would return success/SIGTERM,
but we'd just look at the SIGTERM part.)

If we are doing --wait, let's always propagate the exit code/status from
the child.

While at it, make the documentation useful.

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