* Define `darwin` and `linux` *and* "platform known" pragmas.
* Use per-platform pragmas in `config/__init__.py` selection of implementation.
* Attempt, and fail, to use pragma in `config/darwin.py` to ignore it on
other platforms.
Does what it says on the tin. Moves config implementations out to
individual files, guards those around platforms to make stuff more
reasonably split out.