removing references from backend functions. lookup causes E_NOTICE in php

4.4.0, because tries to return array(). Use of references can't be explained.
Main backend class and file/ldap backends don't use refencences. If it is
used for some reason, other lookup() should have to use references too.
I suspect that reason is performance or it is relict from some different code.
PHP manual says that developers should not use references in order to increase
performance.

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