salloc/sbatch/srun - print warning if both --mem and --mem-per-cpu are set.

Only print a warning for 18.08. If a user has SLURM_MEM_PER_CPU or
SLURM_MEM_PER_NODE environment variables set for some reason this
situation could be happening by accident, and we don't want to prevent
the srun command from launching steps at this point.

Bug 6058.