EZP-21872: Check system user before executing script

Sometimes it can happen, that you forget to execute an eZ CLI script as the webserver user, which can result in files not being created or being created with the wrong owner and/or permission settings.

With this patch, we are able on UNIX systems to check the executing user against a predefined user name and can stop the script execution when the both don't match.

Cheers
:octocat: Jérôme