Run mDNS broadcast asynchronously

Run mDNS in a goroutine so the landing page server starts also in cases
when the outbound IP address can't be determined or the mDNS server
fails to start for another reason.

This is easy to reproduce when the device has no network connectivity
when it starts for the first time. In such case, the landing page errors
with "connect: network is unreachable" and never starts after the cable
is reconnected, leaving port 8123 unreachable until full Core is
downloaded.

Since mDNS is not mission-critical, we shouldn't block the webserver
from starting and just start the mDNS asynchronously.

Author: sairon

main.go

@@ -42,9 +42,11 @@ func main() {
 	http.Handle("/frontend_latest/", staticFiles)
 
 	// Start mDNS broadcast
-	log.Print("Start mDNS broadcast")
-	publishHomeAssistant()
-	defer mdns.Shutdown()
+	go func() {
+		log.Print("Start mDNS broadcast")
+		publishHomeAssistant()
+		defer mdns.Shutdown()
+	}()
 
 	// Run webserver
 	go func() {