Merge branch 'stable-3.1'

install/dist/lib/centos72.lib.php

install/dist/lib/centos_base.lib.php

install/dist/lib/fedora.lib.php

install/install.php

install/lib/install.lib.php

install/lib/installer_base.lib.php

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

install/tpl/config.inc.php.master

install/tpl/helo_access.master

install/tpl/pureftpd_mysql.conf.master

interface/lib/classes/quota_lib.inc.php

interface/lib/classes/session.inc.php

interface/lib/classes/tform.inc.php

interface/lib/classes/tform_base.inc.php

interface/web/admin/lib/lang/de_server_config.lng

interface/web/admin/list/server_ip_map.list.php

interface/web/admin/templates/directive_snippets_edit.htm

interface/web/client/form/client_template.tform.php

interface/web/client/templates/client_del.htm