Because some sites don't have an 'erp5_users' plugin in acl_users, and what
ERP5Site_getAuthenticatedMemberPersonValue does is not specific to 'erp5_users'.

This is done by moving code outside ERP5UserManager class so that it can be
reused. Changes to ERP5UserManager.getUserLogin method are:
- use a transactional cache instead of erp5_content_short:
  - a transactional cache is enough because authenticateCredentials already
    caches its result in erp5_content_short
  - no need to care about empty result from the catalog, which was done using
    _AuthenticationFailure (instead, we simply return an empty list)
  - cache person objects instead of their path
- no need to use SUPER_USER since we use unrestrictedSearchResults

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@37065 20353a03-c40f-0410-a6d1-a30d3c3de9de