BF_Model: add const RETURN_ARRAY for use with $result_type parameter

Prompted during #768

I lose enthusiasm about this, seeing as at least role_model has the same problem as #768 ($result_type not being implemented in user_model).  And simply passing through $result_type won't work, as role_model->find() calls something else that assumes the result should be an object (and not an array).

So maybe it'd be better to remove $result_type.  But here's what I came up with, in case anyone was interested.