Complete chapter7/c in book

Add a keymap but not use it.
add task_tty to schedule which display char.
keyboard_handler only read char from keyboard
and not display it.
Complete chapter7/d in book

use keymap to display normal char.
ignore scan_code with 0xE1 and 0xE0
Complete chapter/e in book

Handle shift alt and ctrl
Complete chapter7/f in book

handle printscreen and pausebreak button.
Both keys has scancodes more than 2.
Complete chapter7/g in book

only print printable char.