* menu.c (Fx_popup_menu): Consolidate versions from xmenu.c,

* menu.c (Fx_popup_menu): Consolidate versions from xmenu.c,
w32menu.c, and nsmenu.m.
Simplify the obsolete case where position is nil.
(cleanup_popup_menu): New function, moved from nsmenu.m.
(struct skp): Remove slot `notreal'.
(single_keymap_panes, keymap_panes): Remove arg `notreal' and adjust callers.
(single_menu_item): Adjust call to parse_menu_item.
(syms_of_menu): Defsubr x-popup-menu.
* menu.h (Vmenu_updating_frame): Consolidate declarations from *menu.c.
(keymap_panes): Don't export any more.
(mouse_position_for_popup, w32_menu_show, ns_menu_show, xmenu_show): Declare.
* keyboard.c (parse_menu_item): Remove arg `notreal'.
(menu_bar_item, read_char_minibuf_menu_prompt): Adjust callers.
* keyboard.h (parse_menu_item): Update declaration.
* xmenu.c (Fx_popup_menu): Remove.
(syms_of_xmenu): Don't defsubr x-popup-menu.
* w32menu.c (Fx_popup_menu): Remove.
(syms_of_w32menu): Don't defsubr x-popup-menu.
* nsmenu.m (cleanup_popup_menu): Remove.
(ns_menu_show): Rename from ns_popup_menu and remove all the code
moved to menu.c's Fx_popup_menu.
(Fx_popup_menu): Remove.
(syms_of_nsmenu): Don't defsubr x-popup-menu, and don't initialize
menu_items (it's done in menu.c already).