bug#7496: 23.2; copy recursive keymap cause crash

[Stefan Monnier]

> When I call `copy-keymap' with a keymap which contains recursive binding,
> the emacs crashed.
> I can reproduce it by following sexp.

> (let ((map (make-sparse-keymap)))
>   (define-key map " " map)
>   (copy-keymap map))

I'm not surprised.  There are many ways to address it:
- try and make sure we better handle the "using up all memory" case
  rather than crashing.  This is very difficult.  We already try to do
  it, but clearly it's not working that well.
- try and detect such cycles and either signal an error or reproduce the
  same cycle in the copy.  We have added such things in several other
  cases, so we should probably do that.
- don't use cyclic keymaps and especially don't copy them.
- don't use copy-keymap, instead: inherit.


        Stefan