greatly improve SIGEV_THREAD timers

calling pthread_exit from, or pthread_cancel on, the timer callback
thread will no longer destroy the timer.

src/internal/pthread_impl.h

@@ -42,6 +42,7 @@ struct pthread {
 		void *pending;
 	} robust_list;
 	int unblock_cancel;
+	int delete_timer;
 };
 
 struct __timer {