[HAVE_NTGUI]: Include w32term.h.

[HAVE_NTGUI]: Include w32term.h.
[HAVE_NTGUI] (KBD_BUFFER_SIZE): Increase for NT window system.
[HAVE_NTGUI] (POLL_FOR_INPUT): Define.
[HAVE_NTGUI] (kbd_buffer_get_event): Enable windowing and menu events.
[HAVE_NTGUI] (lispy_function_keys): Define array for NT.
[HAVE_NTGUI] (make_lispy_event): Enable menu bar events.
[HAVE_NTGUI] (modify_event_symbol): Map symbol to keysym.
[HAVE_NTGUI] (read_char_x_menu_prompt): Display menus.

(FUNCTION_KEY_OFFSET): New macro.
(make_lispy_event): Use FUNCTION_KEY_OFFSET to modify event codes
before applying modifiers.

Use HAVE_WINDOW_SYSTEM instead of testing for specific window systems.