Use record_unwind_protect_ptr to avoid allocation

Use record_unwind_protect_ptr to avoid allocation

* src/term.c (struct tty_pop_down_menu): New type.
(tty_pop_down_menu, tty_menu_show): Use it, along with
record_unwind_protect_ptr, to avoid allocating a Lisp_Misc.
* src/xmenu.c (struct pop_down_menu): New type.
(pop_down_menu, x_menu_show): Use it, likewise.
* src/xterm.c (x_cr_destroy, x_cr_export_frames):
Use record_unwind_protect_pointer to avoid possibly allocating
a Lisp_Misc.