upnp: add option to configure interface for db plugin

Add an option to the UPnP database plugin to configure which interface
is used by upnp to discover servers.

upnp by default selects the first interface that is not loopback, which
in some cases might not be the desired interface. For example if wanting
to access a DLNA server over a VPN connection.

The "interface" option can now be set to the name of the desired
interface to achieve this.

The default behaviour remains unchanged.

src/db/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -39,6 +39,7 @@
 #include "util/ConstBuffer.hxx"
 #include "util/RecursiveMap.hxx"
 #include "util/SplitString.hxx"
+#include "config/Block.hxx"
 
 #include <cassert>
 #include <string>
@@ -76,10 +77,13 @@ class UpnpDatabase : public Database {
 	UpnpClient_Handle handle;
 	UPnPDeviceDirectory *discovery;
 
+	const char* interface;
+
 public:
-	explicit UpnpDatabase(EventLoop &_event_loop) noexcept
+	explicit UpnpDatabase(EventLoop &_event_loop, const ConfigBlock &block) noexcept
 		:Database(upnp_db_plugin),
-		 event_loop(_event_loop) {}
+		 event_loop(_event_loop),
+		 interface(block.GetBlockValue("interface", nullptr)) {}
 
 	static DatabasePtr Create(EventLoop &main_event_loop,
 				  EventLoop &io_event_loop,
@@ -147,15 +151,15 @@ private:
 DatabasePtr
 UpnpDatabase::Create(EventLoop &, EventLoop &io_event_loop,
 		     [[maybe_unused]] DatabaseListener &listener,
-		     const ConfigBlock &) noexcept
+		     const ConfigBlock &block) noexcept
 {
-	return std::make_unique<UpnpDatabase>(io_event_loop);
+	return std::make_unique<UpnpDatabase>(io_event_loop, block);;
 }
 
 void
 UpnpDatabase::Open()
 {
-	handle = UpnpClientGlobalInit(nullptr);
+	handle = UpnpClientGlobalInit(interface);
 
 	discovery = new UPnPDeviceDirectory(event_loop, handle);
 	try {