(struct kbd_translate): New struct, for key translation

(struct kbd_translate): New struct, for key translation
tables private to certain national keyboard layouts.
(struct dos_keyboard_map): Add a pointer to a private translation
table.
(jp_kbd_translate_table): Private table for the Japanese
keyboard.
(jp_keyboard): Japanese keyboard layout.
(it_kbd_translate_table): Private table for the Italian keyboard.
(it_keyboard): Use it_kbd_translate_table.
(keyboard_layout_list): Add the Japanese keyboard.
(dos_rawgetc): Use the keyboard-private translation table, if
available.
(abort) [__DJGPP_MINOR__ >= 2]: Raise SIGABRT.