database: Use auth_user directive as intended

Prior to this commit, the auth_user directive was translated as user
directive. Those directives are not the same.

The user directive expects credentials to be solved either by the userlist.txt
mechanism or by a password provided in the database entry.

The auth_user directive allows the credentials to be solved through querying.

Ultimately, it must be the module's user to decide which to use.

Related #31