removes osfamily dependency, cleanup of internal variable names

With this patch it is possible to use this module with *unknown* OSfamilies without changing the code by providing values to all 'USE_DEFAULTS' cases.

- rename intername variables from default_* to *_default.
- fix a faulty validation for $main_mydestination
- fix usage of $default_main_command_directory instead of $main_command_directory_real