Removed possibility of reinstalling Instances

-Added a new option "Network Filesystem" under System > Server config > Web to toggle on / off the _netdev option for the bind mounts in /etc/fstab

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

Note: The mod_proxy_fcgi that ships with CentOS and Ubuntu does not support sockets yet, so all php-fpm connections are done by IP.

- Create md5 sums of all files at install and update.

- 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.

- changed hhvm handling to match stable branch (FilesMatch in Directory sections) - added changes from stable accordingly