tun/netstack: implement ICMP ping

Provide a PacketConn interface for netstack's ICMP endpoint; netstack
currently only provides EchoRequest/EchoResponse ICMP support, so this
code exposes only an interface for doing ping.

Currently is missing:
- Write deadlines
- Context support

Signed-off-by: Thomas Ptacek <thomas@sockpuppet.org>
[Jason: rework structure, match std go interfaces, add example code]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/netstack/examples/ping_client.go

@@ -0,0 +1,57 @@
+//go:build ignore
+// +build ignore
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2019-2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package main
+
+import (
+	"log"
+	"time"
+
+	"golang.zx2c4.com/go118/netip"
+	"golang.zx2c4.com/wireguard/conn"
+	"golang.zx2c4.com/wireguard/device"
+	"golang.zx2c4.com/wireguard/tun/netstack"
+)
+
+func main() {
+	tun, tnet, err := netstack.CreateNetTUN(
+		[]netip.Addr{netip.MustParseAddr("192.168.4.29")},
+		[]netip.Addr{netip.MustParseAddr("8.8.8.8")},
+		1420)
+	if err != nil {
+		log.Panic(err)
+	}
+	dev := device.NewDevice(tun, conn.NewDefaultBind(), device.NewLogger(device.LogLevelVerbose, ""))
+	dev.IpcSet(`private_key=a8dac1d8a70a751f0f699fb14ba1cff7b79cf4fbd8f09f44c6e6a90d0369604f
+public_key=25123c5dcd3328ff645e4f2a3fce0d754400d3887a0cb7c56f0267e20fbf3c5b
+endpoint=163.172.161.0:12912
+allowed_ip=0.0.0.0/0
+`)
+	err = dev.Up()
+	if err != nil {
+		log.Panic(err)
+	}
+
+	socket, err := tnet.Dial("ping4", "zx2c4.com")
+	if err != nil {
+		log.Panic(err)
+	}
+	const payload = "gopher burrow"
+	socket.SetReadDeadline(time.Now().Add(time.Second * 10))
+	start := time.Now()
+	_, err = socket.Write([]byte(payload))
+	if err != nil {
+		log.Panic(err)
+	}
+	var reply [len(payload)]byte
+	n, err := socket.Read(reply[:])
+	if err != nil || string(reply[:n]) != payload {
+		log.Panic(err)
+	}
+	log.Printf("Ping latency: %v", time.Since(start))
+}