The current API of this callback mixes responsibilities, including:
  * validation
  * transition checking
  * processing (modifying) the configuration object.

These will have to be disentangled piece by piece, so for now, we'll
have "legacy" validate functions as well.

This is an automated commit, generated by this command:

./scripts/maint/rename_c_identifier.py \
        validate_fn_t legacy_validate_fn_t \
        validate_fn legacy_validate_fn