Add virtual beginMulticast(...) stub to UDP class

cores/esp8266/Udp.h

@@ -43,6 +43,7 @@ class UDP: public Stream {
     public:
         virtual ~UDP() {};
         virtual uint8_t begin(uint16_t) =0;	// initialize, start listening on specified port. Returns 1 if successful, 0 if there are no sockets available to use
+        virtual uint8_t beginMulticast(IPAddress, uint16_t) { return 0; }  // initialize, start listening on specified multicast IP address and port. Returns 1 if successful, 0 on failure
         virtual void stop() =0;  // Finish with the UDP socket
 
         // Sending UDP packets

doc/esp8266wifi/udp-class.rst

@@ -26,11 +26,11 @@ Multicast UDP
 
 .. code:: cpp
 
-    uint8_t  beginMulticast (IPAddress interfaceAddr, IPAddress multicast, uint16_t port) 
+    uint8_t  beginMulticast (IPAddress multicast, uint16_t port) 
     virtual int  beginPacketMulticast (IPAddress multicastAddress, uint16_t port, IPAddress interfaceAddress, int ttl=1) 
     IPAddress  destinationIP () 
     uint16_t  localPort ()
 
-The ``WiFiUDP`` class supports sending and receiving multicast packets on STA interface. When sending a multicast packet, replace ``udp.beginPacket(addr, port)`` with ``udp.beginPacketMulticast(addr, port, WiFi.localIP())``. When listening to multicast packets, replace ``udp.begin(port)`` with ``udp.beginMulticast(WiFi.localIP(), multicast_ip_addr, port)``. You can use ``udp.destinationIP()`` to tell whether the packet received was sent to the multicast or unicast address.
+The ``WiFiUDP`` class supports sending and receiving multicast packets on STA interface. When sending a multicast packet, replace ``udp.beginPacket(addr, port)`` with ``udp.beginPacketMulticast(addr, port, WiFi.localIP())``. When listening to multicast packets, replace ``udp.begin(port)`` with ``udp.beginMulticast(multicast_ip_addr, port)``. You can use ``udp.destinationIP()`` to tell whether the packet received was sent to the multicast or unicast address.
 
 For code samples please refer to separate section with `examples <udp-examples.rst>`__ dedicated specifically to the UDP Class.

libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -84,6 +84,11 @@ uint8_t WiFiUDP::begin(uint16_t port)
     return (_ctx->listen(IPAddress(), port)) ? 1 : 0;
 }
 
+uint8_t WiFiUDP::beginMulticast(IPAddress multicast, uint16_t port)
+{
+    return beginMulticast(IP_ADDR_ANY, multicast, port);
+}
+
 uint8_t WiFiUDP::beginMulticast(IPAddress interfaceAddr, IPAddress multicast, uint16_t port)
 {
     if (_ctx) {

libraries/ESP8266WiFi/src/WiFiUdp.h

@@ -47,6 +47,8 @@ class WiFiUDP : public UDP, public SList<WiFiUDP> {
   // Finish with the UDP connection
   void stop() override;
   // join a multicast group and listen on the given port
+  virtual uint8_t beginMulticast(IPAddress interfaceAddr, uint16_t port);
+  // join a multicast group and listen on the given port, using a specific interface address
   uint8_t beginMulticast(IPAddress interfaceAddr, IPAddress multicast, uint16_t port);
 
   // Sending UDP packets