net: move DNSConfigError to a portable file

The type being unavailable on Windows was the only API
signature difference in the net package.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5608043
diff --git a/src/pkg/net/net.go b/src/pkg/net/net.go
index 84cb4fc..79d36a2 100644
--- a/src/pkg/net/net.go
+++ b/src/pkg/net/net.go
@@ -201,3 +201,15 @@
 func (e UnknownNetworkError) Error() string   { return "unknown network " + string(e) }
 func (e UnknownNetworkError) Temporary() bool { return false }
 func (e UnknownNetworkError) Timeout() bool   { return false }
+
+// DNSConfigError represents an error reading the machine's DNS configuration.
+type DNSConfigError struct {
+	Err error
+}
+
+func (e *DNSConfigError) Error() string {
+	return "error reading DNS config: " + e.Err.Error()
+}
+
+func (e *DNSConfigError) Timeout() bool   { return false }
+func (e *DNSConfigError) Temporary() bool { return false }