[PATCH 1/3] MCS: add 'Multi-Category Security' logic for jobs/nodes tagging

- add new configuration parameters MCSPlugin and MCSParameters
  (MCS logic can be disabled/enabled/enabled-on-demand)
- add mcs_label to jobs and nodes related structures
- add --mcs-label option to srun,salloc,sbatch as well
  as MCS information in the various Slurm commands
- initial none/user/group MCS plugins
- add squeue/sinfo optional filtering based on MCS labels