Before we miss yet another beta freeze, how does something like this look? 

It moves `which` into one function which always yields paths. I don't think anyone will approve of adding a dual-function API to solve this problem. I originally tried an approach where the function returned one value and had an option to return an iterator, but nearly everyone I asked found it unacceptable and potentially confusing.

This patch removes the custom pathext stuff seen in other patches. It doesn't seem like it would be that useful to have it, and I'm not sure why you would want to.

Unless anyone is violently opposed, I think we should just go on with it. If we want to add to it, we can add to it in 3.4, but I think something should go in for 3.3. The issue has been open with various patches for over 10 years...