This simplifies code a lot, with added bonus of enabling use of highest-precedence System Preference.
Also, user_name is not used in the local cached function, so move it to cache key instead.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@45678 20353a03-c40f-0410-a6d1-a30d3c3de9de