Currently, it takes the auth_system name as an optional parameter, and
redirects to the existing services. This revision should have the same
behaviour as the previous one, except for login links and the
associated additional redirections.

Summary:
 - Auth_common.Make is now split in two functors: one for declaring
   services (Make) and one for registering them (Register)
 - auth_systems moved to its own signature
 - login and logout are in their own signature
 - new signature ALL_SERVICES