None config

@michaelcahill, I thought we might want this one (as Alex said, it's a cleaner way to get around default configurations in binaries).

I think it's OK, my only concern is it's dangerous for some reason to modify `value.len` in `__config_process_value()`.

Reference #1417.
I thought about that a bit, it's definitely an intrusion on the user's name space.

I thought about taking `__NONE__` or something equally obscure, but couldn't think of anything that didn't hurt my eyes to write down.

And, the places where we want to allow "none" (like compression engines), the problem is there's no reason someone couldn't try and name their compression engine "none", so I don't see a solution there.

Your good idea goes here (or, we can just continue to force users to use empty strings to reset values, it's not a horrible requirement).