Fix synchronization issue at thread start

If a newly spawned thread called getThreadId or getThreadHandle before
the spawning thread finished saving the thread handle, then the
handle/id would be used uninitialized.  This would cause the threading
tests to fail since isCurrentThread would return false, and if Minetest
is built with C++11 support the std::thread object pointer would be
dereferenced while ininitialized, causing a segmentation fault.

This fixes the issue by using a mutex to force the spawned thread to
wait for the spawning thread to finish initializing the thread object.

An alternative way to handle this would be to also set the thread
handle/id in the started thread but this wouldn't work for C++11
builds because there's no way to get the partially constructed object.

src/threading/thread.h

@@ -153,6 +153,7 @@ private:
 	Atomic<bool> m_request_stop;
 	Atomic<bool> m_running;
 	Mutex m_mutex;
+	Mutex m_start_finished_mutex;
 
 #if USE_CPP11_THREADS
 	std::thread *m_thread_obj;