Add support for OpenBSD.

Further allow to set the owner/group of the database and slapd.conf file
via parameters. Default values are given in params.pp. This is because
on OpenBSD, ldap owner/group is _openldap:_openldap. Further, change the
permissions of slapd.conf from 0644 to 0640. There can be sensitive
information in that file, i.e. the rootpw, and this should just not
be world readable.