Tunnel inbound: Fix panic when listening on UDS for Xray's internal services (e.g. API) (#6062)

And API supports listening on UNIX domain socket directly

Completes https://github.com/XTLS/Xray-core/pull/5693

proxy/dokodemo/dokodemo.go

@@ -95,7 +95,7 @@ func (d *DokodemoDoor) Process(ctx context.Context, network net.Network, conn st
 				}
 			}
 		}
-		if dest.Port == 0 {
+		if dest.Port == 0 && port != "" {
 			dest.Port = net.Port(common.Must2(strconv.Atoi(port)))
 		}
 		if d.portMap != nil && d.portMap[port] != "" {