use pigz if available for tar-backups and add definable tmp-dir for zip-backups

install/tpl/server.ini.master

interface/web/admin/form/server_config.tform.php

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

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

interface/web/admin/templates/server_config_server_edit.htm

server/lib/classes/cron.d/500-backup.inc.php

server/lib/classes/cron.d/500-backup_mail.inc.php