config: Move relay config actions into the relay module

This commit:
* moves relay config actions into relay_config,
* moves get_dirportfrontpage() into relay_config,
* adds thin wrappers to make the moved code compile.

No functional changes: the moved code is still enabled,
even if the relay module is disabled. (Some of the checks
are re-ordered, so the order of some warnings may change.)

Part of 32213.