slurmstepd: Move wait-for-parent code into fork_all_tasks

Move the code that waits for parent signal before exec(2) out of
exec_task() and into fork_all_tasks() directly. This makes all
the code that handles the fork-and-wait into slurmstepd/mgr.c,
and allows the exec_wait_child_wait_for_parent() function to
be used in place of explicit read().