devfsctl - Disable ruletab support; use proper working dirs

devfsctl - Disable ruletab support; use proper working dirs

* Disable (temporarily) all ruletab support in devfsctl.

* If a configuration file cannot be found, it is searched after in the
  default directory (/etc/devfs) before erroring out.

* When including a file in some other directory, the working directory
  is temporarily changed to that dir, so that further includes work
  as intended. After each include, the working directory is restored
  to the previous one.

* Make sure that all configuration files are owned by root so the
  system is protected from malicious rule files.