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?
Yes; I deliberately left the old option in the configuration string. Imagine something like "a=(b=c,d=e,f=g)" where we want to upgrade just "d=e" but leave "b=c" and "f=g" alone. In cases like that, pulling out the old option is going to get messy, and I didn't think it was worth the effort.

I deliberately used both input/output parameters because I thought some future use of the function might not already have the configuration string in a WT_ITEM, but I agree that's going to be confusing, I'll switch that to a single parameter.