Basecfg version

@michaelcahill, @agorrod, here's a first attempt at adding versioning and upgrade support to the base configuration file, reference #1244.

Would you please take a look and then let's talk it over?
I thought about that briefly, but couldn't come up with anything that would work on anything other than simple cases.

For example, I think it's likely simple options will upgrade to complex options as we think of additional choices, so we'd need some kind of grammar that would allow us to specify that `feature=boolean` should upgrade to `group=(feature=list)`, and that's when I decided to hand-code it instead.