make WiFi/Ethernet interface compatible with Arduino Ethernet API (#8645)

* make WiFi/Ethernet interface compatible with Arduino Ethernet API
provide some minimaly adapted examples from legacy

* move ethernet compat globals to EthernetCompat.h

* LegacyEthernet: add UDP example

* adjust comments

Co-authored-by: Max Prokhorov <prokhorov.max@outlook.com>

libraries/lwIP_Ethernet/examples/LegacyAdvancedChatServer/LegacyAdvancedChatServer.ino

@@ -0,0 +1,126 @@
+/*
+ Advanced Chat Server
+
+ A more advanced server that distributes any incoming messages
+ to all connected clients but the client the message comes from.
+ To use, telnet to your device's IP address and type.
+ You can see the client's input in the serial monitor as well.
+ Using an Arduino Wiznet Ethernet shield.
+
+ Circuit:
+ * Ethernet Wiznet5500/Wiznet5100/ENC28J60 on esp8266
+
+ created 18 Dec 2009
+ by David A. Mellis
+ modified 9 Apr 2012
+ by Tom Igoe
+ redesigned to make use of operator== 25 Nov 2013
+ by Norbert Truchsess
+
+ */
+
+// specific to esp8266 w/lwIP
+#include <EthernetCompat.h>
+ArduinoWiznet5500lwIP Ethernet(/*SS*/ 16);  // <== adapt to your hardware
+// ArduinoWiznet5100lwIP Ethernet(/*SS*/ 16);  // <== adapt to your hardware
+// ArduinoENC28J60lwIP Ethernet(/*SS*/ 16);  // <== adapt to your hardware
+
+
+
+// Enter a MAC address and IP address for your controller below.
+// The IP address will be dependent on your local network.
+// gateway and subnet are optional:
+byte notNeededButAllowed_mac[] = {
+  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED
+};
+byte* mac = nullptr;  // automatic mac
+IPAddress ip(192, 168, 1, 177);
+IPAddress myDns(192, 168, 1, 1);
+IPAddress gateway(192, 168, 1, 1);
+IPAddress subnet(255, 255, 0, 0);
+
+
+// telnet defaults to port 23
+EthernetServer server(23);
+
+EthernetClient clients[8];
+
+void setup() {
+  // You can use Ethernet.init(pin) to configure the CS pin
+  // Ethernet.init(10);  // Most Arduino shields
+  // Ethernet.init(5);   // MKR ETH shield
+  // Ethernet.init(0);   // Teensy 2.0
+  // Ethernet.init(20);  // Teensy++ 2.0
+  // Ethernet.init(15);  // ESP8266 with Adafruit Featherwing Ethernet
+  // Ethernet.init(33);  // ESP32 with Adafruit Featherwing Ethernet
+  //                     // esp8266 w/lwIP: SS set in Ethernet constructor
+
+  // initialize the Ethernet device
+  Ethernet.begin(mac, ip, myDns, gateway, subnet);
+
+  // Open serial communications and wait for port to open:
+  Serial.begin(9600);
+  while (!Serial) {
+    ;  // wait for serial port to connect. Needed for native USB port only
+  }
+
+  // Check for Ethernet hardware present
+  if (Ethernet.hardwareStatus() == EthernetNoHardware) {
+    Serial.println("Ethernet shield was not found.  Sorry, can't run without hardware. :(");
+    while (true) {
+      delay(1);  // do nothing, no point running without Ethernet hardware
+    }
+  }
+  if (Ethernet.linkStatus() == LinkOFF) {
+    Serial.println("Ethernet cable is not connected.");
+  }
+
+  // start listening for clients
+  server.begin();
+
+  Serial.print("Chat server address:");
+  Serial.println(Ethernet.localIP());
+}
+
+void loop() {
+  // check for any new client connecting, and say hello (before any incoming data)
+  EthernetClient newClient = server.accept();
+  if (newClient) {
+    for (byte i = 0; i < 8; i++) {
+      if (!clients[i]) {
+        Serial.print("We have a new client #");
+        Serial.println(i);
+        newClient.print("Hello, client number: ");
+        newClient.println(i);
+        // Once we "accept", the client is no longer tracked by EthernetServer
+        // so we must store it into our list of clients
+        clients[i] = newClient;
+        break;
+      }
+    }
+  }
+
+  // check for incoming data from all clients
+  for (byte i = 0; i < 8; i++) {
+    if (clients[i] && clients[i].available() > 0) {
+      // read bytes from a client
+      byte buffer[80];
+      int count = clients[i].read(buffer, 80);
+      // write the bytes to all other connected clients
+      for (byte j = 0; j < 8; j++) {
+        if (j != i && clients[j].connected()) {
+          clients[j].write(buffer, count);
+        }
+      }
+    }
+  }
+
+  // stop any clients which disconnect
+  for (byte i = 0; i < 8; i++) {
+    if (clients[i] && !clients[i].connected()) {
+      Serial.print("disconnect client #");
+      Serial.println(i);
+      clients[i].stop();
+    }
+  }
+}