Allow insert and update tuple routing and COPY for foreign tables.

Also enable this for postgres_fdw.

Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
patch series of which this is a part has been reviewed by Amit
Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
and me.  Minor documentation changes to the final version by me.

Discussion: http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f2b9@lab.ntt.co.jp

contrib/file_fdw/input/file_fdw.source

@@ -136,6 +136,11 @@ DELETE FROM agg_csv WHERE a = 100;
 -- but this should be allowed
 SELECT * FROM agg_csv FOR UPDATE;
 
+-- copy from isn't supported either
+COPY agg_csv FROM STDIN;
+12	3.4
+\.
+
 -- constraint exclusion tests
 \t on
 EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_csv WHERE a < 0;