Fix routing reading issues

- Detect VPN gateway properly
- Fix local subnet detection, refers to #188
- Split LocalSubnet from DefaultRoute (2 different routes actually)

internal/firewall/vpn.go

@@ -27,7 +27,7 @@ func (c *configurator) SetVPNConnections(ctx context.Context, connections []mode
 		return nil
 	}
 
-	defaultInterface, _, _, err := c.routing.DefaultRoute()
+	defaultInterface, _, err := c.routing.DefaultRoute()
 	if err != nil {
 		return fmt.Errorf("cannot set VPN connections through firewall: %w", err)
 	}