Implemented: FS#2582 - Add support for SSL bundle certificates (SSLCACertificateFile) in ispconfig apache vhost

- Added missing validator for xfer field in dns zone fom

Fixed:  FS#2066 - Check if problems can occur ins ervices when the default prefix is manually changed
 - Notes:
   once the entry was created or modified after applying this update the current prefix is stored with the entry and kept even if the global prefix changes

- Added check that aps plugin installs package only once on a mirrored system
- Added "IPCCommTimeout  7200" in ispconfig apache vhost.

- Changed check if a domain is unique so that vhosts with identical domains/hostnames are allowed on different IPs and/or servers.

- Bugfix: sys.ini and server.ini: if checkbox is not checked, write default value from tform file into database.

Bugfix: Typo in db_backup path that leads to a crashing daily cron if the backup path does not exist