In postgres_fdw, don't try to ship MULTIEXPR updates to remote server.

In a statement like "UPDATE remote_tab SET (x,y) = (SELECT ...)",
we'd conclude that the statement could be directly executed remotely,
because the sub-SELECT is in a resjunk tlist item that's not examined
for shippability.  Currently that ends up crashing if the sub-SELECT
contains any remote Vars.  Prevent the crash by deeming MULTIEXEC
Params to be unshippable.

This is a bit of a brute-force solution, since if the sub-SELECT
*doesn't* contain any remote Vars, the current execution technology
would work; but that's not a terribly common use-case for this syntax,
I think.  In any case, we generally don't try to ship sub-SELECTs, so
it won't surprise anybody that this doesn't end up as a remote direct
update.  I'd be inclined to see if that general limitation can be fixed
before worrying about this case further.

Per report from Lukáš Sobotka.

Back-patch to 9.6.  9.5 had MULTIEXPR, but we didn't try to perform
remote direct updates then, so the case didn't arise anyway.

Discussion: https://postgr.es/m/CAJif3k+iA_ekBB5Zw2hDBaE1wtiQa4LH4_JUXrrMGwTrH0J01Q@mail.gmail.com

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1191,6 +1191,26 @@ DELETE FROM ft2
   RETURNING 100;
 DELETE FROM ft2 WHERE ft2.c1 > 1200;
 
+-- Test UPDATE with a MULTIEXPR sub-select
+-- (maybe someday this'll be remotely executable, but not today)
+EXPLAIN (verbose, costs off)
+UPDATE ft2 AS target SET (c2, c7) = (
+    SELECT c2 * 10, c7
+        FROM ft2 AS src
+        WHERE target.c1 = src.c1
+) WHERE c1 > 1100;
+UPDATE ft2 AS target SET (c2, c7) = (
+    SELECT c2 * 10, c7
+        FROM ft2 AS src
+        WHERE target.c1 = src.c1
+) WHERE c1 > 1100;
+
+UPDATE ft2 AS target SET (c2) = (
+    SELECT c2 / 10
+        FROM ft2 AS src
+        WHERE target.c1 = src.c1
+) WHERE c1 > 1100;
+
 -- Test UPDATE/DELETE with WHERE or JOIN/ON conditions containing
 -- user-defined operators/functions
 ALTER SERVER loopback OPTIONS (DROP extensions);