Add postgres_fdw contrib module.

There's still a lot of room for improvement, but it basically works,
and we need this to be present before we can do anything much with the
writable-foreign-tables patch.  So let's commit it and get on with testing.

Shigeru Hanada, reviewed by KaiGai Kohei and Tom Lane

doc/src/sgml/runtime.sgml

@@ -1751,7 +1751,7 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
    (<xref linkend="ssl-tcp">). The TCP client must connect using
    <literal>sslmode=verify-ca</> or
    <literal>verify-full</> and have the appropriate root certificate
-   file installed (<xref linkend="libpq-connect">).
+   file installed (<xref linkend="libq-ssl-certificates">).
   </para>
  </sect1>