Revert "os/exec: make StdoutPipe and StderrPipe safe to Close concurrently"

Revert "os/exec: make StdoutPipe and StderrPipe safe to Close concurrently"

This reverts CL 437176.

Reason for revert: broke programs that plumb StdoutPipe from one command to Stdin on another and then call Wait on the former.

os/exec itself uses a type-assertion to *os.File to determine whether to copy stdin using a goroutine or just pass a file descriptor. An early Wait using a *os.File is benign (because closing the pipe doesn't close the child's inherited file descriptor), but an early Wait using a non-*os.File is not.

Updates #50436.

Change-Id: I4a2993e290982834f91696d890dfe77364c0cc50
Reviewed-on: https://go-review.googlesource.com/c/go/+/438695
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>