Fixed Uncaught TypeError: array_merge(): Argument #1 must be of type array, bool given in /usr/local/ispconfig/server/plugins-available/shelluser_jailkit_plugin.inc.php:530

- fixed several issues
- fixed variable typos
- coding styles

- fixed warning in chmod

- moved jailkit functions from bash files to system library

Fix typo in parent user related variable when creating new jailkit shell user. This bug was interfering with the correct functioning of the plugin.

Improved the code that generates the ssh keys for clients and shell users and reorganized it into a central function.

- 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.