Add initial OpenBSD support.

Based on the work of @buzzdeee, but without conflicts with changes in master
Could you restructure this block so the log_dir/run_dir overrides are set in `$_module_os_overrides`, and these parameters are set to `$_module_parameters['log_dir']` and `$_module_parameters['run_dir']`? Like you did with `global_group` and `sites_available_group`