rt: Change the scheme used for terminating the kernel

Instead of joining on the scheduler threads, instead keep a count of active
schedulers. When there are no more schedulers raise a signal for the main
thread to continue.

This will be required once schedulers can be added and removed from the
running kernel.

src/rt/rust_task_thread.cpp

@@ -296,6 +296,7 @@ rust_task_thread::create_task(rust_task *spawner, const char *name,
 
 void rust_task_thread::run() {
     this->start_main_loop();
+    sched->release_task_thread();
 }
 
 #ifndef _WIN32