cmd/{tailscale,tailscaled}: use netstack for subnet routing on Synology

Updates #707
Fixes #451
Fixes tailscale/tailscale-synology#52 (just make it work by default)

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

cmd/tailscale/cli/up.go

@@ -100,9 +100,6 @@ func runUp(ctx context.Context, args []string) error {
 
 	if distro.Get() == distro.Synology {
 		notSupported := "not yet supported on Synology; see https://github.com/tailscale/tailscale/issues/451"
-		if upArgs.advertiseRoutes != "" {
-			return errors.New("--advertise-routes is " + notSupported)
-		}
 		if upArgs.acceptRoutes {
 			return errors.New("--accept-routes is " + notSupported)
 		}

cmd/tailscaled/tailscaled.go

@@ -335,6 +335,9 @@ func shouldWrapNetstack() bool {
 		}
 		return v
 	}
+	if distro.Get() == distro.Synology {
+		return true
+	}
 	switch runtime.GOOS {
 	case "windows", "darwin":
 		// Enable on Windows and tailscaled-on-macOS (this doesn't