Fix minor problems found by static checking.

Fix minor problems found by static checking.

* dispnew.c (save_current_matrix): Omit unnecessary casts.
* dispnew.c (update_frame_with_menu): Mark debug local as used.
* keyboard.c, keyboard.h (Qmouse_movement): Now static.
* keyboard.c (read_menu_command): Remove unused local.
* lisp.h (read_menu_command): New decl.
* menu.c, menu.h (menu_item_width): Arg is now unsigned char *, for
benefit of STRING_CHAR_AND_LENGTH.  All uses changed.
Return ptrdiff_t, not int.
* term.c (tty_menu_struct): 'allocated' member is now ptrdiff_t,
not int, for benefit of xpalloc.
(tty_menu_create, tty_menu_make_room): Simplify by using xzalloc
and xpalloc.
(have_menus_p): Remove; unused.
(tty_menu_add_pane, tty_menu_add_selection): Change signedness of
local char * pointer to pacify STRING_CHAR_AND_LENGTH.
(tty_menu_add_selection, tty_menu_locate, tty_meny_destroy):
Now static.
(save_and_enable_current_matrix): Omit unnecessary casts.
(read_menu_input): Omit local extern decl (now in lisp.h).
Don't access uninitialized storage if mouse_get_xy fails.
(tty_menu_activate): Mark local as initialized, for lint.
(tty_menu_activate, tty_meny_show): Remove unused locals.