Logtalk and Prolog built-in predicates can no longer be used as messages

This change helps avoiding code portability issues due to different back-end Prolog compilers providing different sets of built-in predicates. In particular, it makes definitions for the `forward/1` unknown message handler work as expected across back-end Prolog compilers when some of them happen to provide a built-in predicate with the same predicate indicator as an otherwise unknown message sent to an object.