Fix errors in Atari keymap (mostly in keypad, help and undo keys).

Patch provided on debian-68k ML by Andreas Schwab <schwab@linux-m68k.org>,
keymap array size and unhandled scancode limit adjusted to 0x73 by me.
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>