Use a prefix for all wither-local config vars.

[slothbear]

Might be gold-plating, but it's tedious to look through the many config vars for the ones that wither sets/uses. There's lots of conf for add-ons and platforms and such (PG, ?).

We already have some good prefixes like DO_ and DNSSIMPLE_. Maybe I just need a wiki page to sort them out for me.