module: Better safeguard authdir_mode_v3() if dirauth module is disabled

We need to safeguard somehow the authdir_mode_v3() so it can NEVER return true if the dirauth module is disabled.

One option here is to move this function to the dirauth module and NOP it by returning false if the module is not enabled.

An other option is to #ifdef HAVE_MODULE_DIRAUTH in the function directly. I'm kind of less enthusiastic about it but maybe it is better, dunno yet.