Code refactoring.
add getchar() system call.
add readline() system call, debugging now!
readline() system call  work now!
add set_status() system call!
add fields to task_t.
rewrite exec, no debug yet!
rewrite sys_exec() system call, and fix bug of phy_free()
Merge branches 'p3_ck3' and 'rw_exec' into p3_ck3