Apply modified version of bug #52747: mdns resp: separate the announce function and add netifapi support for it

Changes made by me:
- Move all error handling code into mdns_resp_announce() so it can be safely used by external code
- Remove mdns_resp_netif_settings_changed() because it is the same as mdns_resp_announce() after my changes
- Declare #define for a "thread-safe" version of mdns_resp_announce in mdns.h instead of netifapi.h - I don't want to intermix netif API with APPs

src/include/lwip/apps/mdns.h

@@ -66,7 +66,24 @@ err_t mdns_resp_del_service(struct netif *netif, s8_t slot);
 
 err_t mdns_resp_add_service_txtitem(struct mdns_service *service, const char *txt, u8_t txt_len);
 
-void mdns_resp_netif_settings_changed(struct netif *netif);
+void mdns_resp_announce(struct netif *netif);
+
+/**
+ * @ingroup mdns
+ * Announce IP settings have changed on netif.
+ * Call this in your callback registered by netif_set_status_callback().
+ * No need to call this function when LWIP_NETIF_EXT_STATUS_CALLBACK==1,
+ * this handled automatically for you.
+ * @param netif The network interface where settings have changed.
+ */
+#define mdns_resp_netif_settings_changed(netif) mdns_resp_announce(netif)
+ 
+/** @ingroup mdns
+ * To trigger announces from non-TCPIP threads
+ * Be sure to \#include lwip/netifapi.h when using this macro
+ * @see mdns_resp_announce()
+ */
+#define mdnsapi_mdns_resp_announce(netif)   netifapi_netif_common(netif, NULL, mdns_resp_announce)
 
 #endif /* LWIP_MDNS_RESPONDER */