The previous implementation of net.Pipe was just a thin wrapper around
io.Pipe and did not wrap any of the io.Pipe errors as net.Errors.
As a result of Hyrum's law, users have come to depend on the fact that
net.Pipe returns io.ErrClosedPipe when the pipe is closed.
Thus, we preserve this behavior to avoid regressing such use cases.

Change-Id: I06b387877b944c1c08527601f58983872b7557b4
Reviewed-on: https://go-review.googlesource.com/81777
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>