Ensure that the contents of a holdable cursor don't depend on out-of-line

Ensure that the contents of a holdable cursor don't depend on out-of-line
toasted values, since those could get dropped once the cursor's transaction
is over.  Per bug #4553 from Andrew Gierth.

Back-patch as far as 8.1.  The bug actually exists back to 7.4 when holdable
cursors were introduced, but this patch won't work before 8.1 without
significant adjustments.  Given the lack of field complaints, it doesn't seem
worth the work (and risk of introducing new bugs) to try to make a patch for
the older branches.