Change the childFiles field to a local variable, since it was
populated during Start and (as far as I can determine) has no purpose
after Start returns.
Rename closeAfterStart and closeAfterWait to childIOFiles and
parentIOPipes respectively. That makes their contents clearer, and also
helps to clarify what should happen on error (when, for example, Wait
shouldn't be called at all).
Use a deferred call instead of individual calls to close child (and,
if necessary, pipe) FDs after Start. That helps to clarify the
invariants around when they are closed, and also makes the function a
bit more robust for future refactoring.
Also nil out the slices containing the file closers so that they can
be collected earlier.
This CL is intended as a pure refactor in preparation for #50436.