The network connection dies differently from the server's
perspective on (some) Windows when the client goes away. Match
on the common prefix (common to Unix and Windows) instead of
the network error part.

Fixes #7456

LGTM=josharian
R=golang-codereviews, josharian
CC=alex.brainman, golang-codereviews, iant
https://golang.org/cl/70010050