Introduce new mDNS API, add mDNS client example

src/dns.h

@@ -3,6 +3,13 @@
 #include "net.h"
 #include "str.h"
 
+#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.
@@ -38,18 +45,26 @@ 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 *);
@@ -58,3 +73,4 @@ 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);