- changed language import to use mb_substr

Fixed language file import problem: https://www.howtoforge.com/community/threads/import-language-issue.70355

- Added warning in the interface when a path for a shelluser is set that is outside of the website docroot.
- Added security settings feature to allow the root user of a server to control most aspects of whet the admin user of the controlpanel is allowed to do in system settings. This is especially useful for managed severs where the ispconfig admin user and the root user of the server are different persons.

Added the configuration variable $conf['demo_mode'] in the config.inc.php file. It disables the newly introduced demo mode which limits certain actions in the interface part of ispconfig: the demo mode can be used to run the ispconfig interface part as online demo.

Added a Checkbox to the language import form to ignore the version of the language file.

- Added dutch translation (many thanks to Hans!)

- Changed shell of website linux user to /bin/false