Get available port for UNET socket

UnetAvailablePort.md

using System.Net;
using System.Net.Sockets;
using UnityEngine.Networking;

var address = IPAddress.Parse("0.0.0.0");
int port;
using (var tempSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp)) {
    tempSocket.Bind(new IPEndPoint(address, port: 0));
    port = ((IPEndPoint) tempSocket.LocalEndPoint).Port;
}

var topology = new HostTopology(new ConnectionConfig(), maxDefaultConnections: 100);
var hostId = NetworkTransport.AddHost(topology, port, address.ToString());