cmd/tailscaled, wgengine{,/netstack}: add netstack hybrid mode, add to Windows

For #707

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

wgengine/userspace_test.go

@@ -187,14 +187,3 @@ func BenchmarkGenLocalAddrFunc(b *testing.B) {
 	})
 	b.Logf("x = %v", x)
 }
-
-func TestIsNetstack(t *testing.T) {
-	e, err := NewUserspaceEngine(t.Logf, Config{})
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer e.Close()
-	if !IsNetstack(e) {
-		t.Errorf("IsNetstack = false; want true")
-	}
-}