I'm taking Charles-François' review comments here.

> 1. since fwalk() uses O(depth directory tree) file descriptors, we might run out
> of FD on really deep directory hierarchies. It shouldn't be a problem in
> practise

Should I mention it in the docs? The old one uses recursion and we don't warn about the stack too...

> 2. there is a slight API change, since the API exposes the function that
> triggered the failure. I don't think there's a lot a of code that depends on
> this, but it's definitely a change

I was pondering whether I should "fake" the method names as they pretty much map: listdir instead of fwalk and unlink instead of unlink at… what do you all think about that?