validate job gres on slurmctld restart

this new function adds logic to validate that a job's gres request is
still valid after slurmctld restart or reconfiguration. specifically
if a job allocation includes GRES associated with specific Files then
kill the job if the GRES count changes (say the job is allocated GPUs
and the slurmctld configuration changes to remove those GPUs, so the
job and node bitmaps are different sizes).