control/controlhttp: allow client and server to communicate over WebSockets

We can't do Noise-over-HTTP in Wasm/JS (because we don't have bidirectional
communication), but we should be able to do it over WebSockets. Reuses
derp WebSocket support that allows us to turn a WebSocket connection
into a net.Conn.

Updates #3157

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

control/controlhttp/client.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !js
+// +build !js
+
 // Package controlhttp implements the Tailscale 2021 control protocol
 // base transport over HTTP.
 //
@@ -40,21 +43,6 @@ import (
 	"tailscale.com/types/key"
 )
 
-const (
-	// upgradeHeader is the value of the Upgrade HTTP header used to
-	// indicate the Tailscale control protocol.
-	upgradeHeaderValue = "tailscale-control-protocol"
-
-	// handshakeHeaderName is the HTTP request header that can
-	// optionally contain base64-encoded initial handshake
-	// payload, to save an RTT.
-	handshakeHeaderName = "X-Tailscale-Handshake"
-
-	// serverUpgradePath is where the server-side HTTP handler to
-	// to do the protocol switch is located.
-	serverUpgradePath = "/ts2021"
-)
-
 // Dial connects to the HTTP server at addr, requests to switch to the
 // Tailscale control protocol, and returns an established control
 // protocol connection.