In followup changes, we'll move OpError around from the netFD layer to
the Conn layer for fixing #4856. Before doing that, this change makes
netFD of Plan 9 match netFD for POSIX platforms to avoid conflict.
Change-Id: Iea7632716d48722a1758e52effefec964a3a9442
Reviewed-on: https://go-review.googlesource.com/8990 Reviewed-by: David du Colombier <0intro@gmail.com> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>