os/exec: split parent I/O pipes by whether they are pumped by user code or internal goroutines
The pipes pumped by goroutines can be closed as soon as their
respective goroutines are done.
The pipes pumped by user code, however, are documented to be closed in
Wait. When we add the WaitDelay field, it isn't obvious that we should
terminate the user-pumped pipes when the WaitDelay expires, since Wait
itself isn't going to wait for those user-controlled goroutines to
complete.
(It's a bit more complicated than that because the documentation
currently states that Wait must not be called while the pipes are
being read — but it isn't obvious to me that that advice is entirely
correct.)