(check_memory_limits): New fn; most code from morecore_with_warning, but
only checks limits, doesn't do any work.
(memory_warnings): Set __after_morecore_hook to check_memory_limits;
don't set __morecore.
(memory_warnings): Renamed from malloc_init.
Don't set lim_data or warnlevel. Use start_of_data if start is 0.
[!emacs]: Don't include config.h or lisp.h;
instead, use stddef.h. Define POINTER, SIZE, EXCEEDS_LISP_PTR.
(morecore_with_warning): Use EXCEEDS_LISP_PTR.