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

src/pkg/net/dnsconfig.go

@@ -17,19 +17,6 @@ type dnsConfig struct {
 	rotate   bool     // round robin among servers
 }
 
-var dnsconfigError error
-
-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 }
-
 // See resolv.conf(5) on a Linux machine.
 // TODO(rsc): Supposed to call uname() and chop the beginning
 // of the host name to get the default search domain.

src/pkg/net/net.go

@@ -201,3 +201,15 @@ type UnknownNetworkError string
 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 }