wgengine/magicsock, tstest/natlab: start hooking up natlab to magicsock

Also adds ephemeral port support to natlab.

Work in progress.

Pairing with @danderson.

types/nettype/nettype.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package nettype defines an interface that doesn't exist in the Go net package.
+package nettype
+
+import (
+	"context"
+	"net"
+)
+
+// PacketListener defines the ListenPacket method as implemented
+// by net.ListenConfig, net.ListenPacket, and tstest/natlab.
+type PacketListener interface {
+	ListenPacket(ctx context.Context, network, address string) (net.PacketConn, error)
+}
+
+// Std implements PacketListener using the Go net package's ListenPacket func.
+type Std struct{}
+
+func (Std) ListenPacket(ctx context.Context, network, address string) (net.PacketConn, error) {
+	var conf net.ListenConfig
+	return conf.ListenPacket(ctx, network, address)
+}