slurmstepd: Fix race in run_script_as_user

As reported by Sam Lang on slurm-dev, task_epilog scripts are not
held before exec, and thus there is a race condition between when
the task_epilog is launched and slurmstepd calls slurm_container_add()
during which the task_epilog script could either run to completion, or
launch other processes that escape any job container defined by
configuration.

Use the new "exec_wait" api to have the child wait before exec just
as is done in fork_all_tasks.

Based on an original idea by Sam Lang <samlang@gmail.com>.