Avoid test generating core file on segv

We do not want to look at the core file, so avoid generating it
and then having to manually clear it later.
Fix slurmstepd race condition causing abort

Fix slurmstepd race condition when separate threads are reading and
modifying the job's environment, which can result in the slurmstepd failing
with an invalid memory reference. Observed at shutdown when trying
to run the task epilog and trying to read the env var:
SLURM_STEP_KILLED_MSG_NODE_ID
Add typecast to prevent build warning
Merge branch 'slurm-2.6'

Conflicts:
	src/slurmd/slurmstepd/task.c