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?
Does this deliberately leave the old option in the configuration string? Both uses of this function pass the ```config``` parameter as cbuf->buf, could the method take a single parameter to make it clearer that the method is operating on a single string?