Adjust tuplesort.c based on the fact that we never use the OS's qsort().

Our own qsort_arg() implementation doesn't have the defect previously
observed to affect only QNX 4, so it seems sufficiently to assert that
it isn't broken rather than retesting.  Also, update a few comments to
clarify why it's valuable to retain a tie-break rule based on CTID
during index builds.

Peter Geoghegan, with slight tweaks by me.