udp.h, udp.c, sockets.c: Changes for "#20503 IGMP Improvement". Add IP_MULTICAST_IF option in socket API, and a new field "multicast_ip" in "struct udp_pcb" (for netconn and raw API users), only if LWIP_IGMP=1. Add getsockopt processing for IP_MULTICAST_TTL and IP_MULTICAST_IF.

src/include/lwip/udp.h

@@ -77,7 +77,12 @@ struct udp_pcb {
   u8_t flags;
   /* ports are in host byte order */
   u16_t local_port, remote_port;
-  
+
+#if LWIP_IGMP
+  /* outgoing network interface for multicast packets */
+  struct ip_addr multicast_ip;
+#endif /* LWIP_IGMP */
+
 #if LWIP_UDPLITE
   /* used for UDP_LITE only */
   u16_t chksum_len_rx, chksum_len_tx;