qsort improvements

[Florian Weimer]

* Paul Eggert:

> Since _quicksort is rarely used nowadays (only if malloc fails), I
> think we should pass on that. Any value in improving performance in
> the rare cases when _quicksort is used would likely be outweighed by
> the maintenance hassles of the fancier swapping code.

Do we keep calling malloc from qsort?  It's somewhat surprising that we
do that.

Thanks,
Florian