Improved ispconfig.vhost for apache so that the sethandler directive does not affect other software which is accessed trough aliases.

ISPConfig itself does not read nor interpret this header, but there might be other
server components like PHP that might interact with it so I'll disable it just to be sure.

First part of fix for issue #3896 - Unknown column 'default_mailserver' in 'field list' when saving client limits main template

This reverts commit 008ef889.

Removed ubuntu1604.lib.php which is not required anymore.

- Introduced confid in $distver array to separate the config and class for a distribution in the installer.
- Cleaned up the dist detection.