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/nodes/copyfuncs.c

@@ -905,9 +905,6 @@ _copyPlanRowMark(PlanRowMark *from)
 	COPY_SCALAR_FIELD(markType);
 	COPY_SCALAR_FIELD(noWait);
 	COPY_SCALAR_FIELD(isParent);
-	COPY_SCALAR_FIELD(ctidAttNo);
-	COPY_SCALAR_FIELD(toidAttNo);
-	COPY_SCALAR_FIELD(wholeAttNo);
 
 	return newnode;
 }