implemented changes suggested by @matthijskooijman

 - renaming _sock to _lastReturnedSocket
 - move increment line up

The changes make that the clients returned by `EthernetServer::available()` are more
evenly distributed over the 4 sockets of the w5100 chip.

In the previous impelmenatation there was a problem if there is a client
connected to socket number 0 and the client is permanently sending
data. In that case `EthernetServer::available()` always returned that client,
even if other clients are connected and have data available.

Author: sauerburger

libraries/Ethernet/src/EthernetServer.cpp

@@ -52,22 +52,21 @@ EthernetClient EthernetServer::available()
 {
   accept();
 
-  // increment _sock to avoid returning the same socket again
-  _sock = (_sock + 1) % MAX_SOCK_NUM;
-
   for (int i = 0; i < MAX_SOCK_NUM; i++) {
-    EthernetClient client(_sock);
-    if (EthernetClass::_server_port[_sock] == _port) {
+    // increment _lastReturnedSocket to avoid returning the same socket again
+    _lastReturnedSocket = (_lastReturnedSocket + 1) % MAX_SOCK_NUM;
+
+    EthernetClient client(_lastReturnedSocket);
+    if (EthernetClass::_server_port[_lastReturnedSocket] == _port) {
       uint8_t s = client.status();
       if (s == SnSR::ESTABLISHED || s == SnSR::CLOSE_WAIT) {
         if (client.available()) {
-          // doesn't always pick the lowest numbered socket, because of _sock +
-          // 1 at the beginning
+          // doesn't always pick the lowest numbered socket, because of
+          // _lastReturnedSocket + 1 at the beginning
           return client;
         }
       }
     }
-    _sock = (_sock + 1) % MAX_SOCK_NUM;
   }
 
   return EthernetClient(MAX_SOCK_NUM);

libraries/Ethernet/src/EthernetServer.h

@@ -10,7 +10,7 @@ public Server {
 private:
   uint16_t _port;
   void accept();
-  int _sock = -1; 
+  int _lastReturnedSocket = -1; 
 public:
   EthernetServer(uint16_t);
   EthernetClient available();