Added INSTALLER_UPDATE constant in update.php to allow easier detection if installer run is an update or new installation.

Disabled sql-mode check in updater.

fixed code styling (bit of a perfectionist) - Looks like someone accidentially removed an if clause here before and fixing killed styling.

Implementation of detection override to configure webserver wheenver the panel is installed on the host regardless of user's preference

updater: db-query working, requests working. This will be my MR and hopefully fixes ispconfig/ispconfig3#4163 and hopefully also fixes ispconfig/ispconfig3#4050

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

- Fixed: FS#3710 - Bug with PHP-FPM configuration since 3.0.5.4 Patch 4
- Fixed wrong function calls in system lib.

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