8284: Fix confusion between %proc and %jobstep.

8284: Fix confusion between %proc and %jobstep.

$proc{$pid}->{jobstep} is an index into @jobstep
$proc{$pid}->{jobstepname} is the name we told srun to use
$proc{$pid}->{killtime} is a deadline when we should kill the process
$jobstep[$jobstepid]->{stderr_at} is the time of last stderr received

We were mistakenly using $proc->{$pid}->{stderr_at}, which was always
undef and therefore always less than $last_squeue_check. This resulted
in jobs being killed as "slurm orphans" when the real reason they
hadn't been returned by waitpid() was that we hadn't finished
consuming their stderr yet.