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>

ipn/ipnserver/server_test.go

@@ -56,7 +56,7 @@ func TestRunMultipleAccepts(t *testing.T) {
 		}
 	}
 
-	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0, nil)
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0)
 	if err != nil {
 		t.Fatal(err)
 	}