Support new config method.

Refined code.

if_windows.go

@@ -0,0 +1,12 @@
+// +build windows
+
+package water
+
+import "errors"
+
+func newDev(config Config) (ifce *Interface, err error) {
+	if config.DeviceType != TAP && config.DeviceType != TUN {
+		return nil, errors.New("unknown device type")
+	}
+	return openDev(config)
+}