* Makefile.in: Compile menu.c.

* Makefile.in: Compile menu.c.

* lisp.h: Declare syms_of_menu.

* emacs.c (main): Call syms_of_menu.

* keyboard.h: Relocate platform-independent menu definitions from
xmenu.c.

* menu.c: New file.  Relocate platform-independent menu
definitions from xmenu.c.

* xmenu.c: Remove platform-independent menu definitions.
(menu_items menu_items_inuse, menu_items_allocated)
(menu_items_used, menu_items_n_panes)
(menu_items_submenu_depth): Move to keyboard.h.
(init_menu_items, finish_menu_items, unuse_menu_items)
(discard_menu_items, restore_menu_items, save_menu_items)
(grow_menu_items, push_submenu_start, push_submenu_end)
(push_left_right_boundary, push_menu_pane, push_menu_item)
(keymap_panes, single_keymap_panes, single_menu_item)
(list_of_panes, list_of_items, find_and_call_menu_selection)
(xmalloc_widget_value, free_menubar_widget_value_tree)
(parse_single_submenu, digest_single_submenu)
(update_submenu_strings): Move to menu.c.