Added MDNS and driver support for multicast

src/dns.h

@@ -36,3 +36,5 @@ 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);