net: use better error messages on windows

Fixes #4320.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6810064
diff --git a/src/pkg/net/net.go b/src/pkg/net/net.go
index 9ebcdbe..d6563e0 100644
--- a/src/pkg/net/net.go
+++ b/src/pkg/net/net.go
@@ -221,6 +221,8 @@
 
 var errTimeout error = &timeoutError{}
 
+var errClosing = errors.New("use of closed network connection")
+
 type AddrError struct {
 	Err  string
 	Addr string