upnp: expose interface configuration on UpnpInit2()

Adds the Interface Name as an argument to the *Init functions to make it
possible to select which interface is used by upnp to detect servers.

Currently "nullptr" is passed in to let the upnp library select an
interface, as before.

src/db/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -155,7 +155,7 @@ UpnpDatabase::Create(EventLoop &, EventLoop &io_event_loop,
 void
 UpnpDatabase::Open()
 {
-	handle = UpnpClientGlobalInit();
+	handle = UpnpClientGlobalInit(nullptr);
 
 	discovery = new UPnPDeviceDirectory(event_loop, handle);
 	try {