Introduce new mDNS API, add mDNS client example

mongoose.h

@@ -1658,10 +1658,8 @@ 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_MDNS_REQ,   // mDNS request                 struct mg_mdns_req *
+  MG_EV_MDNS_RESP,  // mDNS response                struct mg_mdns_resp *
   MG_EV_USER        // Starting ID for user events
 };
 
@@ -2952,6 +2950,13 @@ size_t mg_mqtt_next_prop(struct mg_mqtt_message *, struct mg_mqtt_prop *,
 
 
 
+#define MG_DNS_RTYPE_A  1
+#define MG_DNS_RTYPE_PTR  12
+#define MG_DNS_RTYPE_TXT  16
+#define MG_DNS_RTYPE_AAAA  28
+#define MG_DNS_RTYPE_SRV  33
+
+
 // Mongoose sends DNS queries that contain only one question:
 // either A (IPv4) or AAAA (IPv6) address lookup.
 // Therefore, we expect zero or one answer.
@@ -2987,18 +2992,26 @@ struct mg_dnssd_record {
   uint16_t port;            // SRV record port
 };
 
-// mDNS request
+// mDNS request and response data structs passed to event handlers
 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_str respname;  // actual name to use in response
   struct mg_addr addr;
   bool is_listing;
   bool is_resp;
   bool is_unicast;
 };
 
+struct mg_mdns_resp {
+  struct mg_dns_rr *rr;
+// TODO(scaprile )struct mg_str srvcproto; struct mg_str txt; uint16_t port; ?
+  struct mg_str name;
+  struct mg_addr addr;
+// TODO(scaprile); bool has_A; bool has_PTR; bool has_SRV; bool has_TXT; ?
+};
+
 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 *);
@@ -3007,6 +3020,7 @@ size_t mg_dns_parse_rr(const uint8_t *buf, size_t len, size_t ofs,
 
 struct mg_connection *mg_mdns_listen(struct mg_mgr *mgr, mg_event_handler_t fn,
                                      void *fn_data);
+bool mg_mdns_query(struct mg_connection *, const char *, unsigned int);