auth: Change passdb_default_fields and passdb_override_fields to be strlist

auth: Change passdb_default_fields and passdb_override_fields to be strlist

Now that the %variable expansion is done by settings lookups, they had to
be moved to their own settings structs that are looked up at appropriate
times. Especially the override_fields must be looked up only after the
passdb lookup has finished, since it can refer to extra fields returned by
it.