Export SLURM_ environment variables from sbatch, even if not --exported

Without this change, when the sbatch --export option is used, many
Slurm environment variables are not set unless explicitly exported.
Add support for SBATCH_RESERVATION environment variable
Add support for SALLOC_RESERVATION and SLURM_RESERVATION for salloc and srun

This permits default reservation names to be more easily managed
Merge branch 'slurm-2.5'