control/controlhttp/controlhttpserver: split out Accept to its own package

Otherwise all the clients only using control/controlhttp for the
ts2021 HTTP client were also pulling in WebSocket libraries, as the
server side always needs to speak websockets, but only GOOS=js clients
speak it.

This doesn't yet totally remove the websocket dependency on Linux because
Linux has a envknob opt-in to act like GOOS=js for manual testing and force
the use of WebSockets for DERP only (not control). We can put that behind
a build tag in a future change to eliminate the dep on all GOOSes.

Updates #1278

Change-Id: I4f60508f4cad52bf8c8943c8851ecee506b7ebc9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/noise_test.go

@@ -15,7 +15,7 @@ import (
 	"time"
 
 	"golang.org/x/net/http2"
-	"tailscale.com/control/controlhttp"
+	"tailscale.com/control/controlhttp/controlhttpserver"
 	"tailscale.com/internal/noiseconn"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
@@ -201,7 +201,7 @@ func (up *Upgrader) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return nil
 	}
 
-	cbConn, err := controlhttp.AcceptHTTP(r.Context(), w, r, up.noiseKeyPriv, earlyWriteFn)
+	cbConn, err := controlhttpserver.AcceptHTTP(r.Context(), w, r, up.noiseKeyPriv, earlyWriteFn)
 	if err != nil {
 		up.logf("controlhttp: Accept: %v", err)
 		return