config: Move dirauth config into the dirauth module

This commit:
* creates feature/dirauth/dirauth_config.[ch],
* moves the dirauth config code into them,
* copies some macros from src/app/config.c
  (we'll refactor them later in 29211), and
* adds thin wrappers to make the moved code compile.

No functional changes: the moved code is still enabled,
even if the dirauth module is disabled.

Part of 32213.