Add multicast support to loopif

This commit adds support to send and receive multicast on the loopback
netif by enabling IGMP via NETIF_FLAG_IGMP

This commit also introduces an LwIP configuration option,
LWIP_LOOPIF_MCAST, to control the behavior and it defaults to off

src/core/netif.c

@@ -126,6 +126,9 @@ netif_loopif_init(struct netif *netif)
 #endif
 #if LWIP_IPV6
   netif->output_ip6 = netif_loop_output_ipv6;
+#endif
+#if LWIP_LOOPIF_MCAST
+  netif->flags |= NETIF_FLAG_IGMP;
 #endif
   return ERR_OK;
 }