exchange_registers(), modified thread_control calls seem to work

- Fixed do_mmap() so that it returns mapped address, and various bugs.
- A child seems to fork with new setup, but with incorrect return value.
  Need to use and test exregs() for fork + clone.
- Shmat searches an unmapped area if input arg is invalid, do_mmap()
  should do this.

src/generic/scheduler.c

@@ -155,7 +155,7 @@ static inline void sched_rq_remove_task(struct ktcb *task)
 	task->rq = 0;
 }
 
-static inline void sched_init_task(struct ktcb *task)
+void sched_init_task(struct ktcb *task)
 {
 	INIT_LIST_HEAD(&task->rq_list);
 	task->ticks_left = TASK_TIMESLICE_DEFAULT;
@@ -226,7 +226,7 @@ void sched_notify_resume(struct ktcb *task)
 /* NOTE: Might as well just set need_resched instead of full yield.
  * This would work on irq context as well. */
 /* Same as resume, but also yields. */
-int sched_resume_task(struct ktcb *task)
+void sched_resume_task(struct ktcb *task)
 {
 	sched_notify_resume(task);
 	sched_yield();