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

Fixed: Issue #3820 v3.1 Shell user wrong default Base Dir in GUI results to corrupt shell user home directory creation.

- SSH users cannot be deleted if the web site uses PHP-FPM ("userdel: user ssh_test is currently used by process 2890"). Fixed this by stopping PHP-FPM before user deletion and starting it again afterwards.

- 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.

Fixed a bug in Opensuse installer (permissions of squirrelmail files)
Fixed a issue in smtp-authentication on courier-imap systems when custom usernames are used.

Reverted SVN trunk back to revision 3938 to remove changes in default theme an a replacement of the ispconfig logo.

- Added backup size to web backups