Fixes #3997 - Add backup of /etc/{init.d,default}/rudder-agent in preinst and restore action in check-rudder-agent cron file

Fixes #3997 - Add backup of /etc/{init.d,default}/rudder-agent in preinst and restore action in check-rudder-agent cron file

See http://www.rudder-project.org/redmine/issues/3997