Use terminal-specific hooks to display menus.
* termhooks.h (struct terminal): New field menu_show_hook.
* menu.h (<anonymous enum>): Bit flags for menu hooks.
(x_menu_show, w32_menu_show, ns_menu_show, tty_menu_show):
Adjust prototypes.
* menu.c (Fx_popup_menu): Use bit flags and menu_show_hook.
* nsmenu.m (ns_menu_show):
* w32menu.c (w32_menu_show):
* xmenu.c (x_menu_show):
* term.c (tty_menu_show): Adjust to use bit flags.
(set_tty_hooks): Set menu_show_hook.
* xterm.c (x_create_terminal):
* nsterm.m (ns_create_terminal):
* msdos.c (initialize_msdos_display):
* w32term.c (w32_create_terminal): Likewise.