Transfer GPU file information to slurmstepd

Add logic to cache GPU file information (bitmap index mapping to device
file number) in the slurmd daemon and transfer that information to the
slurmstepd whenever a job step is initiated. This is needed to set the
appropriate CUDA_VISIBLE_DEVICES environment variable value when the
devices are not in strict numeric order (e.g. some GPUs are skipped).
Based upon work by Nicolas Bigaouette.