bug#11934: 24.1; provide variable for pp.el to control max display width

[Drew Adams]

> `pp-buffer'.  That would be the function to extend
> (Drew, wanne give it a try?).

Sorry, I really don't have the time.  But I welcome improvements. ;-)
 
> But of course handling this problem is not trivial: we can't distinguish
> "simple lists" from code, and what might look good for the first might
> look weird for a macro call with a special lisp-indent-function value
> (e.g. imagine a `defun' call with a line break inserted just after
> "defun").

Yes.

> Another challenge is what to do if we have already
> nearly exhausted the width limit.

Not sure what you mean, there.