Fix Lua scripting synchronization

For several years now, the lua script lock has been completely broken.
This commit fixes the main issue (creation of a temporary rather than
scoped object), and fixes a subsequent deadlock issue caused by
nested script API calls by adding support for recursive mutexes.

src/threading/mutex.h

@@ -49,7 +49,7 @@ DEALINGS IN THE SOFTWARE.
 class Mutex
 {
 public:
-	Mutex();
+	Mutex(bool recursive=false);
 	~Mutex();
 	void lock();
 	void unlock();