Capture return from _spawn_prolog_stepd, and run at correct time.

- Do not re-launch the extern step if !first_run.
- Move before _remove_job_running_prolog() to make sure the
  extern step has at least a slight head start before steps can
  launch. Note that this only ensures the stepd has started, but
  not that the stepd is finished it's own internal setup - further
  work in slurmstepd would be required to remove this race completely.

Bug 4264.