Fix to handling of DCG goals in meta predicates

I've fixed a small bug in sandbox.pl -- the code that handles meta predicates was using expand_phrase/2 incorrectly. NB, there is another use of expand_phrase/2 in this file; this looks OK to me, but I'm not 100% sure about it.

There is another issue with the documentation, in that safe_meta_predicate/1 is declared multifile, but not documented. Also, there is safe_meta/1 which is not multifile. I'm not sure why there are 3 different ways to declare a safe meta predicate.

Finally, apologies if this pull request contains some old commits -- after pulling recent changes from the upstream, I could not figure out how to get Github to forget the changes from my earlier pull request.