Introduce DNS-SD using mDNS

mongoose.h

@@ -1641,6 +1641,10 @@ enum {
   MG_EV_MQTT_OPEN,  // MQTT CONNACK received        int *connack_status_code
   MG_EV_SNTP_TIME,  // SNTP time received           uint64_t *epoch_millis
   MG_EV_WAKEUP,     // mg_wakeup() data received    struct mg_str *data
+  MG_EV_MDNS_A,     // mDNS A record request        struct mg_mdns_req *
+  MG_EV_MDNS_PTR,   // mDNS PTR record request      struct mg_mdns_req *
+  MG_EV_MDNS_SRV,   // mDNS SRV record request      struct mg_mdns_req *
+  MG_EV_MDNS_TXT,   // mDNS TXT record request      struct mg_mdns_req *
   MG_EV_USER        // Starting ID for user events
 };
 
@@ -2950,13 +2954,33 @@ struct mg_dns_rr {
   uint16_t alen;    // Address length
 };
 
+
+// DNS-SD response record
+struct mg_dnssd_record {
+  struct mg_str srvcproto;  // service.proto, service name
+  struct mg_str txt;        // TXT record contents
+  uint16_t port;            // SRV record port
+};
+
+// mDNS request
+struct mg_mdns_req {
+  struct mg_dns_rr *rr;
+  struct mg_dnssd_record *r;
+  struct mg_str reqname;        // requested name in RR
+  struct mg_str respname;       // actual name in response
+  struct mg_addr addr;
+  bool is_listing;
+  bool is_resp;
+  bool is_unicast;
+};
+
 void mg_resolve(struct mg_connection *, const char *url);
 void mg_resolve_cancel(struct mg_connection *);
 bool mg_dns_parse(const uint8_t *buf, size_t len, struct mg_dns_message *);
 size_t mg_dns_parse_rr(const uint8_t *buf, size_t len, size_t ofs,
                        bool is_question, struct mg_dns_rr *);
                        
-struct mg_connection *mg_mdns_listen(struct mg_mgr *mgr, char *name);
+struct mg_connection *mg_mdns_listen(struct mg_mgr *mgr, mg_event_handler_t fn, void *fn_data);