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'.

issues #935 と #964 の議論を読み、その後、名前が「bitcoin」から「elements」に変更された configure および make のビルドファイルを調べてください。これらの変更を upstream と比較し、どの名前が user-facing でどの名前が内部用かを特定してください。不要な差異を減らしつつ user-facing の名前を変更しない、焦点を絞った提案または調整済みの変更ができれば完了です。

索引モデルが issue の本文から書いたものです。