Disallow replacing joins with scans in problematic cases.

Commit e7cb7ee14, which introduced the infrastructure for FDWs and
custom scan providers to replace joins with scans, failed to add support
handling of pseudoconstant quals assigned to replaced joins in
createplan.c, leading to an incorrect plan without a gating Result node
when postgres_fdw replaced a join with such a qual.

To fix, we could add the support by 1) modifying the ForeignPath and
CustomPath structs to store the list of RestrictInfo nodes to apply to
the join, as in JoinPaths, if they represent foreign and custom scans
replacing a join with a scan, and by 2) modifying create_scan_plan() in
createplan.c to use that list in that case, instead of the
baserestrictinfo list, to get pseudoconstant quals assigned to the join;
but #1 would cause an ABI break.  So fix by modifying the infrastructure
to just disallow replacing joins with such quals.

Back-patch to all supported branches.

Reported by Nishant Sharma.  Patch by me, reviewed by Nishant Sharma and
Richard Guo.

Discussion: https://postgr.es/m/CADrsxdbcN1vejBaf8a%2BQhrZY5PXL-04mCd4GDu6qm6FigDZd6Q%40mail.gmail.com

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -640,6 +640,9 @@ SELECT t1c1, avg(t1c1 + t2c1) FROM (SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
+-- join with pseudoconstant quals, not pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1 AND CURRENT_USER = SESSION_USER) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
 
 -- non-Var items in targetlist of the nullable rel of a join preventing
 -- push-down in some cases