This hides the method Group::add_group() on both MacOS and OpenBSD since
the function putgrent(), which is essential for add_group() to work, is
not available on these OSes.
This adds the Core::Group C++ abstraction to ease interaction with the
group entry database, as well as represent the Group entry.
Core::Group abstraction currently contains the following functionality:
- Add a group entry - 'Core::Group::add_group()'
