Set consistent checkpoint directory

I've noticed that parameter JobCheckpointDir has a (from my point of view) inconsistent behavior.

* in sbatch executions, it is exported as CWD
* in srun it is also exported as CWD,
* except when it is manually set with "--checkpoint-dir=dir". If so, that value is exported.
* value defined in slurm.conf is, as far as I know, never read.

I have created this small patch to correct that behaviour. Now it is exported with the value configured on slurm.conf. If nothing is set, returned value is the defined on common/read_config.h,