Introduce new mDNS API, add mDNS client example

tutorials/udp/mdns-client/main.c

@@ -0,0 +1,37 @@
+// Copyright (c) 2022 Cesanta Software Limited
+// All rights reserved
+
+#include "mongoose.h"
+
+char s_theone[] = "Server.local";
+
+static void mdns_ev_handler(struct mg_connection *c, int ev, void *ev_data) {
+  if (ev == MG_EV_MDNS_RESP) {  // is this the one ?
+    struct mg_mdns_resp *resp = (struct mg_mdns_resp *) ev_data;
+    if (mg_strcasecmp(mg_str(s_theone), resp->name) == 0)
+      MG_INFO(("%.*s is: %M", resp->name.len, resp->name.buf, mg_print_ip, &resp->addr));
+  }
+  (void) c;
+}
+
+int main(void) {
+  uint32_t response_ip = inet_addr("192.168.69.11");
+  struct mg_mgr mgr;
+  static struct mg_connection *c;
+  mg_log_set(MG_LL_DEBUG);  // Set log level
+  mg_mgr_init(&mgr);        // Initialise event manager
+
+  // Desired name must NOT have any dots in it, nor a domain
+  c = mg_mdns_listen(&mgr, mdns_ev_handler, "Mongoose");  // Start mDNS server
+  if (c == NULL) return 1;
+  // if not using our built-in TCP/IP stack, pass the IP address you want to
+  // use as a response, this depends on your underlying TCP/IP stack and number
+  // of interfaces available
+  memcpy(c->data, &response_ip, sizeof(response_ip));
+  if (!mg_mdns_query(c, s_theone, MG_DNS_RTYPE_A)) return 1;
+
+  for (;;) mg_mgr_poll(&mgr, 1000);  // Event loop
+
+  mg_mgr_free(&mgr);
+  return 0;
+}