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.
It's the quoted version I'm concerned about, not the name.

I'll check the callers, but I'm uncomfortable saying that there is no way to ever pass the string "none" to anything.  We can pass the strings "true" or "false" with quoting, for contrast...