Modified _doChangeUser to leave password unchanged if the 'password' parameter is None.  _changeUser now always accepts "password == confirm == None", and sets 'password' to None if "password == 'password' and confirm == 'confirm'".

This gives Users an unambiguous protocol for both DTML and Python code to change roles/domains without touching the password.  editUser.dtml has a somewhat magical protocol, which users need not worry about.