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.

include/l4/generic/scheduler.h

@@ -35,6 +35,7 @@ static inline struct ktcb *current_task(void)
 					 | SCHED_FL_SUSPEND)
 
 void sched_runqueue_init(void);
+void sched_init_task(struct ktcb *task);
 void sched_start_task(struct ktcb *task);
 void sched_resume_task(struct ktcb *task);
 void sched_suspend_task(struct ktcb *task);