Disable mod_security for the ispconfig controlpanel vhost as mod_security produces false positive warnings.

Fixed: FS#1183 - remove server_id part from conditional stamenet in postfix and courier / dovecot queries.

Added alternative is_group detection function in system.inc.php. The new function is currently disabled.

Added better error handling to vlogger script in case that the mysql database connection is not available.

- Added a missing language string for the website form.

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- Added a robots.txt file in the root directory of the ispconfig interface to prevent it from being spidered by search engine robots that follow the robots excusion standard.
- Fixed a error message in the installer.