Fix another oversight in CustomScan patch.

execCurrent.c's search_plan_tree() must recognize a CustomScan on the
target relation.  This would only be helpful for custom providers that
support CurrentOfExpr quals, which is probably a bit far-fetched, but
it's not impossible I think.  But even without assuming that, we need
to recognize a scanned-relation match so that we will properly throw
error if the desired relation is being scanned with both a CustomScan
and a regular scan (ie, self-join).

Also recognize ForeignScanState for similar reasons.  Supporting WHERE
CURRENT OF on a foreign table is probably even more far-fetched than
it is for custom scans, but I think in principle you could do it with
postgres_fdw (or another FDW that supports the ctid column).  This
would be a back-patchable bug fix if existing FDWs handled CurrentOfExpr,
but I doubt any do so I won't bother back-patching.

src/backend/executor/execCurrent.c

@@ -258,13 +258,15 @@ search_plan_tree(PlanState *node, Oid table_oid)
 	switch (nodeTag(node))
 	{
 			/*
-			 * scan nodes can all be treated alike
+			 * Relation scan nodes can all be treated alike
 			 */
 		case T_SeqScanState:
 		case T_IndexScanState:
 		case T_IndexOnlyScanState:
 		case T_BitmapHeapScanState:
 		case T_TidScanState:
+		case T_ForeignScanState:
+		case T_CustomScanState:
 			{
 				ScanState  *sstate = (ScanState *) node;