Removed possibility of reinstalling Instances

- Directive snippets can now be made available to clients; clients can select an available directive snippet for a website - the appropriate snippet will then be written to the vhost configuration file. This is useful for example on nginx where clients have no access to the Options tab of a website.

- Changed tform_actions.inc.php to always provide oldDataRecord array for updates even when dbhistory = off.

Use dovecot or maildrop in default-cases
Switching to LMTP and back included in server-postfix-plugin
validator added to disable switching uid-mapping when already created mailusers or in multiserver-environment

FS#3632 - Adding websites with remote API is broken in 3.0.5.4p3 release.
Missed to commit one file for: FS#3634 - Add option to disable statistics for a website

- 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.

additionally fixes this for client_update

- Added some cleanup code