cmd/derper, derp/derphttp: add websocket support

Updates #3157

Change-Id: I337a919a3b350bc7bd9af567b49c4d5d6616abdd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/websocket.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2021 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.
+
+//go:build linux || js
+// +build linux js
+
+package derphttp
+
+import (
+	"context"
+	"log"
+	"net"
+
+	"nhooyr.io/websocket"
+	"tailscale.com/derp/wsconn"
+)
+
+func init() {
+	dialWebsocketFunc = dialWebsocket
+}
+
+func dialWebsocket(ctx context.Context, urlStr string) (net.Conn, error) {
+	c, res, err := websocket.Dial(ctx, urlStr, &websocket.DialOptions{
+		Subprotocols: []string{"derp"},
+	})
+	if err != nil {
+		log.Printf("websocket Dial: %v, %+v", err, res)
+		return nil, err
+	}
+	log.Printf("websocket: connected to %v", urlStr)
+	return wsconn.New(c), nil
+}