rt: Detach pthreads before exiting

Joinable pthreads need to be either joined or detached and we no
longer join with the scheduler threads.

src/rt/rust_task_thread.cpp

@@ -319,6 +319,7 @@ rust_task_thread::create_task(rust_task *spawner, const char *name,
 
 void rust_task_thread::run() {
     this->start_main_loop();
+    detach();
     sched->release_task_thread();
 }