Introduce an API to store arbitrary data pointers in struct netif

Let MDNS to use the new API
TODO: AutoIP, DHCP

src/include/lwip/netif.h

@@ -38,7 +38,6 @@
 #define LWIP_HDR_NETIF_H
 
 #include "lwip/opt.h"
-#include "lwip/apps/mdns_opts.h"
 
 #define ENABLE_LOOPBACK (LWIP_NETIF_LOOPBACK || LWIP_HAVE_LOOPIF)
 
@@ -59,9 +58,6 @@ struct autoip;
 #if LWIP_IPV6_DHCP6
 struct dhcp6;
 #endif /* LWIP_IPV6_DHCP6 */
-#if LWIP_MDNS_RESPONDER
-struct mdns_host;
-#endif
 
 #ifdef __cplusplus
 extern "C" {
@@ -259,10 +255,9 @@ struct netif {
   /** the AutoIP client state information for this netif */
   struct autoip *autoip;
 #endif
-#if LWIP_MDNS_RESPONDER
-  /** Interface-specific info for multicast DNS */
-  struct mdns_host *mdns;
-#endif /* LWIP_MDNS_RESPONDER */
+#if LWIP_NUM_NETIF_CLIENT_DATA > 0
+  void* client_data[LWIP_NUM_NETIF_CLIENT_DATA];
+#endif
 #if LWIP_IPV6_AUTOCONFIG
   /** is this netif enabled for IPv6 autoconfiguration */
   u8_t ip6_autoconfig_enabled;
@@ -431,6 +426,10 @@ void netif_poll_all(void);
 
 err_t netif_input(struct pbuf *p, struct netif *inp);
 
+#if LWIP_NUM_NETIF_CLIENT_DATA > 0
+u8_t netif_alloc_client_data_id(void);
+#endif
+
 #if LWIP_IPV6
 /** @ingroup netif */
 #define netif_ip_addr6(netif, i)  ((const ip_addr_t*)(&((netif)->ip6_addr[i])))

src/include/lwip/opt.h

@@ -1465,6 +1465,14 @@
 #if !defined LWIP_NETIF_TX_SINGLE_PBUF || defined __DOXYGEN__
 #define LWIP_NETIF_TX_SINGLE_PBUF             0
 #endif /* LWIP_NETIF_TX_SINGLE_PBUF */
+
+/**
+ * LWIP_NUM_NETIF_CLIENT_DATA: Number of clients that may store
+ * per data in client_data member array of struct netif.
+ */
+#if !defined LWIP_NUM_NETIF_CLIENT_DATA || defined __DOXYGEN__
+#define LWIP_NUM_NETIF_CLIENT_DATA            (LWIP_DHCP + LWIP_AUTOIP)
+#endif
 /**
  * @}
  */