SLURM_IMMEDIATE_EXIT to SLURM_EXIT_IMMEDIATE

    to permit user control over exit codes so application exit codes can be
    distiguished from those generated by SLURM. SLURM_ERROR_EXIT specifies the
    exit code when a SLURM error occurs. SLURM_IMMEDIATE_EXIT specifies the 
    exit code when the --immediate option is specified and resources are not
    available. Any other non-zero exit code would be that of the application
    run by SLURM.

env var rather than always being 1. This enables a user to distinguish between
slurm failures and application failures

    of slurmctld rather than that of the command.

    within the job's memory allocation and enforce limits).