auth: note that we never emit authuser "cookies" for the default user

The only place where we set "authuser" in the session is in log_in_user,
which is called only by the internal auth system and by auth plugins.
The internal auth system cannot log a user in as the default user,
because the default user doesn't have a password (and cannot have a
password assigned). Auth plugins cannot log a user in as the default
user, because the user doesn't have the right extern_type. As such, it's
a bug if log_in_user is ever called with the default user (which this
commit documents with an assert).

This realization makes the is_authenticated field of the authuser cookie
redundant, as it's always True. It also emphasizes that is_default_user
and is_authenticated are mutually exclusive.