thread/Thread: add debug attribute "inside_handle"

This attribute shall be used only for IsInside() to make this safe
against a race condition described in #188:

> There is no requirement on the implementation that the ID of the
> created thread be available before the newly created thread starts
> executing.

http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_create.html):

This means that on some pthread implementations (e.g. Haiku), the
assert(thread.IsInside()) could fail.

Closes #188

src/thread/Thread.cxx

@@ -86,6 +86,10 @@ Thread::ThreadProc(void *ctx)
 {
 	Thread &thread = *(Thread *)ctx;
 
+#ifndef NDEBUG
+	thread.inside_handle = pthread_self();
+#endif
+
 	thread.Run();
 
 	return nullptr;