slurmd: Do not launch job step during job prolog

If a job step request comes in while the slurm prolog is running,
slurmd will happily launch the job step. This means that a user
could run code before the prolog is complete, which could cause
strange errors or in some cases a security issue. Instead return
an error (EINPROGRESS for now) and do not allow job steps to run
during prolog.