o add new slurm_cred_t type for slurm job credentials. This replaces

   the old slurm_job_credential_t, which was one cred per job, to a
   one cred per *job step* scheme. Creds are now limited lifetime and
   cannot be replayed (cred is invalidated once used).

   removed old implementation signature/credential_utils.[ch] and
   replaced with slurm_cred.[ch].