[patch] Create cleanups.[ch]

[Doug Evans]

I wrote:

> Since cleanups are a big enough source of issues, I want to separate them out.
> This patch moves the core cleanup API into its own file.
> It makes no other changes.
> I have at least one more patch to go, but I want to get this done first.
> [...]
> +/* WARNING: The result of the "make cleanup" routines is not the intuitive
> +   choice of being a handle on the just-created cleanup.  Instead it is an
> +   opaque handle of the cleanup mechanism and represents all cleanups created
> +   from that point onwards.
> +   The result is guaranteed to be non-NULL though.  */

Heh, I'm getting ahead of myself. :-)
Ignore the last sentence there.
The wording of the previous sentence is fine with me, but one may
prefer it to say something along the lines of being the handle of
the previous cleanup ("old_chain") or something like that, or say
nothing at all other than being an opaque handle.