output of uid_to_string() and gid_to_string().

Also change message slightly (e.g. "forcing job termination" to
"job abort in progress") and don't print them continuously, only once
per second.

    a job step is cancelled, another is started immediately (before the 
    first one completely terminates) and ports are reused. 
    NOTE: This change requires that SLURM be updated on all nodes of the
    cluster at the same time. There will be no impact upon currently running
    jobs (they will ignore the jobid/stepid at the end of the message).