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
clean code.
add wait() and vanish() system call, test no pass yet!
clean code!
fix bug of sys_exec()
sys_wait() and sys_vanish() partly works now.
add zombie_init_task_list
change protection bit of page directory!
shell run and fork and exec work now.
shell runs correctly now.
rename hello_world.c to hw.c
split heap.c module from mm.c