Revert misguided change to postgres_fdw FOR UPDATE/SHARE code.

In commit 462bd95705, I changed postgres_fdw
to rely on get_plan_rowmark() instead of get_parse_rowmark().  I still
think that's a good idea in the long run, but as Etsuro Fujita pointed out,
it doesn't work today because planner.c forces PlanRowMarks to have
markType = ROW_MARK_COPY for all foreign tables.  There's no urgent reason
to change this in the back branches, so let's just revert that part of
yesterday's commit rather than trying to design a better solution under
time pressure.

Also, add a regression test case showing what postgres_fdw does with FOR
UPDATE/SHARE.  I'd blithely assumed there was one already, else I'd have
realized yesterday that this code didn't work.

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -237,6 +237,39 @@ SELECT * FROM ft1 t1 WHERE t1.c1 = 101 AND t1.c6 = '1' AND t1.c7 >= '1';
  101 |  1 | 00101 | Fri Jan 02 00:00:00 1970 PST | Fri Jan 02 00:00:00 1970 | 1  | 1          | foo
 (1 row)
 
+-- with FOR UPDATE/SHARE
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = 101 FOR UPDATE;
+                                                   QUERY PLAN                                                   
+----------------------------------------------------------------------------------------------------------------
+ LockRows
+   Output: c1, c2, c3, c4, c5, c6, c7, c8, t1.*
+   ->  Foreign Scan on public.ft1 t1
+         Output: c1, c2, c3, c4, c5, c6, c7, c8, t1.*
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" = 101)) FOR UPDATE
+(5 rows)
+
+SELECT * FROM ft1 t1 WHERE c1 = 101 FOR UPDATE;
+ c1  | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  
+-----+----+-------+------------------------------+--------------------------+----+------------+-----
+ 101 |  1 | 00101 | Fri Jan 02 00:00:00 1970 PST | Fri Jan 02 00:00:00 1970 | 1  | 1          | foo
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = 102 FOR SHARE;
+                                                  QUERY PLAN                                                   
+---------------------------------------------------------------------------------------------------------------
+ LockRows
+   Output: c1, c2, c3, c4, c5, c6, c7, c8, t1.*
+   ->  Foreign Scan on public.ft1 t1
+         Output: c1, c2, c3, c4, c5, c6, c7, c8, t1.*
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" = 102)) FOR SHARE
+(5 rows)
+
+SELECT * FROM ft1 t1 WHERE c1 = 102 FOR SHARE;
+ c1  | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  
+-----+----+-------+------------------------------+--------------------------+----+------------+-----
+ 102 |  2 | 00102 | Sat Jan 03 00:00:00 1970 PST | Sat Jan 03 00:00:00 1970 | 2  | 2          | foo
+(1 row)
+
 -- aggregate
 SELECT COUNT(*) FROM ft1 t1;
  count 

contrib/postgres_fdw/postgres_fdw.c

@@ -822,7 +822,7 @@ postgresGetForeignPlan(PlannerInfo *root,
 	}
 	else
 	{
-		PlanRowMark *rc = get_plan_rowmark(root->rowMarks, baserel->relid);
+		RowMarkClause *rc = get_parse_rowmark(root->parse, baserel->relid);
 
 		if (rc)
 		{
@@ -835,18 +835,15 @@ postgresGetForeignPlan(PlannerInfo *root,
 			 * complete information about, and (b) it wouldn't work anyway on
 			 * older remote servers.  Likewise, we don't worry about NOWAIT.
 			 */
-			switch (rc->markType)
+			switch (rc->strength)
 			{
-				case ROW_MARK_EXCLUSIVE:
+				case LCS_FORKEYSHARE:
-				case ROW_MARK_NOKEYEXCLUSIVE:
+				case LCS_FORSHARE:
-					appendStringInfoString(&sql, " FOR UPDATE");
-					break;
-				case ROW_MARK_SHARE:
-				case ROW_MARK_KEYSHARE:
 					appendStringInfoString(&sql, " FOR SHARE");
 					break;
-				default:
+				case LCS_FORNOKEYUPDATE:
-					/* nothing needed */
+				case LCS_FORUPDATE:
+					appendStringInfoString(&sql, " FOR UPDATE");
 					break;
 			}
 		}

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -151,6 +151,11 @@ SELECT * FROM ft1 WHERE false;
 -- with WHERE clause
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE t1.c1 = 101 AND t1.c6 = '1' AND t1.c7 >= '1';
 SELECT * FROM ft1 t1 WHERE t1.c1 = 101 AND t1.c6 = '1' AND t1.c7 >= '1';
+-- with FOR UPDATE/SHARE
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = 101 FOR UPDATE;
+SELECT * FROM ft1 t1 WHERE c1 = 101 FOR UPDATE;
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = 102 FOR SHARE;
+SELECT * FROM ft1 t1 WHERE c1 = 102 FOR SHARE;
 -- aggregate
 SELECT COUNT(*) FROM ft1 t1;
 -- join two tables