Added new callback to signal steps when not running as srun

RELEASE_NOTES

slurm/slurm.h.in

src/api/step_launch.c

src/plugins/launch/poe/launch_poe.c

src/plugins/launch/runjob/launch_runjob.c

src/plugins/launch/slurm/launch_slurm.c

src/plugins/switch/nrt/libpermapi/shr_64.c

src/srun/libsrun/launch.c

src/srun/libsrun/launch.h

src/srun/srun.c