Redo how flags are sent to the stepd from the slurmctld.

src/api/signal.c

src/common/slurm_protocol_defs.h

src/common/slurm_protocol_pack.c

src/common/stepd_api.c

src/common/stepd_api.h

src/slurmctld/job_mgr.c

src/slurmd/slurmd/req.c

src/slurmd/slurmstepd/req.c