rc.subr: Move the sleep in wait_for_pids

[des]

Summary

Instead of sleeping while constructing the list of PIDs to wait for,
sleep briefly after pwait to give init a chance to reap the processes
that just terminated before we loop around and start probing them.

PR: 289630

[0mp]

Why did we sleep in the for loop in the first place before?

[des]

In D52619#1203159, @0mp wrote:

Why did we sleep in the for loop in the first place before?

I believe the intent was the same, it was just poorly implemented, and probably never thoroughly tested with more than one PID. I added automated tests, which are now failing intermittently because once in a great while, after pwait returns, we manage to kill -0 the process that just terminated before init gets around to collecting it.