Fix #4808 Load the existing keys without new line, so array_search can find the old ones. Unset a key from existing_keys only if a match is false and avoid false being treated as 0

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.

This option can be enabled under System > Server config and is named "Web folder protection".