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'm concerned about saying nobody can ever use the quoted string `"none"` in a config string.  We don't try to support this for `true` or `false`, but then we don't expect them to be used in all the same places, so I understand why something like this is necessary.

Let me have a closer look at the callers: maybe we can layer this cleanly on top in the places that expect `none` rather than changing the handling of every string value.