Fix PlanRowMark/ExecRowMark structures to handle inheritance correctly.

In an inherited UPDATE/DELETE, each target table has its own subplan,
because it might have a column set different from other targets.  This
means that the resjunk columns we add to support EvalPlanQual might be
at different physical column numbers in each subplan.  The EvalPlanQual
rewrite I did for 9.0 failed to account for this, resulting in possible
misbehavior or even crashes during concurrent updates to the same row,
as seen in a recent report from Gordon Shannon.  Revise the data structure
so that we track resjunk column numbers separately for each subplan.

I also chose to move responsibility for identifying the physical column
numbers back to executor startup, instead of assuming that numbers derived
during preprocess_targetlist would stay valid throughout subsequent
massaging of the plan.  That's a bit slower, so we might want to consider
undoing it someday; but it would complicate the patch considerably and
didn't seem justifiable in a bug fix that has to be back-patched to 9.0.

src/backend/optimizer/prep/prepunion.c

@@ -1292,10 +1292,6 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 			newrc->markType = oldrc->markType;
 			newrc->noWait = oldrc->noWait;
 			newrc->isParent = false;
-			/* junk attrs for children are not identified yet */
-			newrc->ctidAttNo = InvalidAttrNumber;
-			newrc->toidAttNo = InvalidAttrNumber;
-			newrc->wholeAttNo = InvalidAttrNumber;
 
 			root->rowMarks = lappend(root->rowMarks, newrc);
 		}