Added support for remote connection by name via Pentair servers

This brings along proper password encryption and validation for the login process (which is still not needed for local connections).

You can run the remote connection tests by passing a system name and password as 2 arguments (so the system name needs to be quoted, e.g.: `test "Pentair: xx-xx-xx" 1234`)

Also updated to latest .NET Core and C# versions while I was at it.

FindUnits.cs

@@ -30,7 +30,7 @@ namespace ScreenLogicConnect
                 await udpClient.SendAsync(broadcastData, broadcastData.Length, new IPEndPoint(IPAddress.Broadcast, multicastPort));
 
                 var buf = await udpClient.ReceiveAsync().TimeoutAfter(TimeSpan.FromSeconds(1));
-                if (buf != null)
+                if (buf != null && buf.RemoteEndPoint != null)
                 {
                     var findServerResponse = new EasyTouchUnit(buf);
                     if (findServerResponse.isValid)