Improved backup in `userzip` mode

ISPConfig creates a zip file in 2 commands: first it creates a zip and puts there files that belong to user group, after that it updates created archive and adds there files that belong to apache/nginx user.

This PR makes a complete archive in a single command.

added MR: Review Pending label

changed milestone to %3.2

removed MR: Review Pending label

merged

mentioned in commit 90de6774