Ways to reduce divergence with upstream, to ease merging

There is some discussion about this in various places (#935, #964), but I wanted to make a dedicated issue to propose things I'm running into while reviewing merges in #935.

One thing I've talked to someone about, I forget who -- right now in our build stuff (configure/make), a lot of things were renamed from 'bitcoin' to 'elements', but also a lot of things were not. I think _probably_ the thing that would make this easiest for future maintenance would be to minimize the diff from upstream, by ONLY using 'elements' in places where it's user-facing (file/directory/target names), and revert all the names of purely-internal variables (some of which were changed, and some of which were not) back to 'bitcoin'.