Notify srun and ctld when unkillable stepd exits

Commits f18390e8 and eed76f85 modified the stepd so that if the
stepd encountered an unkillable step timeout that the stepd would just
exit the stepd. If the stepd is a batch step then it would reply back
to the controller with a non-zero exit code which will drain the node.
But if an srun allocation/step were to get into the unkillable step
code, the steps wouldn't let the waiting srun or controller know about
the step going away -- leaving a hanging srun and job.

This patch enables the stepd to notify the waiting sruns and the ctld of
the stepd being done and drains the node for srun'ed alloction and/or
steps.

Bug 5164