8.0 res users rco

Fix the unexpected returned value of default_get(): a many2one field will return False instead of a dict.
Suppress warnings when reading reified group fields on res.users, and some extra cleanup.

You don't expect it, but it does :-)
The method check_field_access_rights() used by read() determines which fields to read. The case fields=[] will return all fields (the ones for which you have access rights).