sched: add current_task(), current_thread(), and preempt_disable/enable()

sched/task.c

@@ -77,3 +77,8 @@ task_t *task_from_pid(unsigned int pid)
 	spin_unlock_irqrestore(&task_list_lock, flags);
 	return t;
 }
+
+task_t *current_task(void)
+{
+	return current_thread()->tr_parent;
+}