Questionable coding in orderedsetaggs.c

In ordered_set_startup() sorts are initialised in non-randomAccess mode
(tuplesort_begin_heap() and ~datum(), last argument).

The use of tuplesort_skip_tuples() feels very like a random access to
me.  I think it doesn't fail because the only use (and implementation)
is to skip forwards; if backwards were tried (as the interface permits)
external sorts would fail because multiple tapes are present for
FINALMERGE.
-- 
Cheers,   Jeremy