Move userspace code to sub-package in proxy.

Moves the userspace code in proxy to a sub-package and adds the
ProxyProvider interface.

This is in preparation for landing an implementation of
https://github.com/GoogleCloudPlatform/kubernetes/issues/3760, which
will mostly be in another sub package for iptables.

pkg/proxy/userspace/udp_server.go

@@ -0,0 +1,54 @@
+/*
+Copyright 2014 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package userspace
+
+import (
+	"fmt"
+	"net"
+)
+
+// udpEchoServer is a simple echo server in UDP, intended for testing the proxy.
+type udpEchoServer struct {
+	net.PacketConn
+}
+
+func (r *udpEchoServer) Loop() {
+	var buffer [4096]byte
+	for {
+		n, cliAddr, err := r.ReadFrom(buffer[0:])
+		if err != nil {
+			fmt.Printf("ReadFrom failed: %v\n", err)
+			continue
+		}
+		r.WriteTo(buffer[0:n], cliAddr)
+	}
+}
+
+func newUDPEchoServer() (*udpEchoServer, error) {
+	packetconn, err := net.ListenPacket("udp", ":0")
+	if err != nil {
+		return nil, err
+	}
+	return &udpEchoServer{packetconn}, nil
+}
+
+/*
+func main() {
+	r,_ := newUDPEchoServer()
+	r.Loop()
+}
+*/