cmd/tailscaled, wgengine: remove --fake, replace with netstack

And add a --socks5-server flag.

And fix a race in SOCKS5 replies where the response header was written
concurrently with the copy from the backend.

Co-authored with Naman Sood.

Updates #707
Updates #504

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

wgengine/userspace_test.go

@@ -84,7 +84,7 @@ func TestNoteReceiveActivity(t *testing.T) {
 }
 
 func TestUserspaceEngineReconfig(t *testing.T) {
-	e, err := NewFakeUserspaceEngine(t.Logf, 0, nil)
+	e, err := NewFakeUserspaceEngine(t.Logf, 0)
 	if err != nil {
 		t.Fatal(err)
 	}