Revert "ERP5Security.ERP5LoginUserManager: enumerateUsers() should not include Persons without any Login."

This reverts commit caefbddd.

patches/DCWorkflow: use 'user/getIdOrUserName' as workflow actor expression instead of 'user/getId'.

so that ID is used for ERP5 user case, that should never change, and User Name is used for system users
like Anonymous User or System Processes, without changing all workflow definitions.

This transition is now available when Person has no validated Login document.
Person's reference is now set only when it is not yet set.

Before it raised an exception if called with id=(A, B) where A has Login and B has no Login.

Simplified rework of ERP5 User Manager without module-level callables and
implementing more of PAS API.
Implement IAuthenticationPlugin API better, so PAS API becomes usable for
ERP5-based authentication.

use getId() instead of getUserName() where user is supposed to be neither Anonyous User nor System Processes.

so that getId() is used for ERP5 users and getUserName() is used for special users
like Anonyous User or System Processes whose id is None.

Allows being compatible with future, improved PAS plugin features.
Also, drop a few useless imports.