Generate a suitable default-value for --data-local
A fresh installation should have a suitable default value for --data-local. Should also work for the uninstalled version. We can reuse the same settings for both cases.
(RM-134 from redmine: created on 2011-08-06 by Lukasz Gromanowski, , closed on 2012-09-13 by nobody, relates #168 (closed))