pgsql: When WCOs are present, disable direct foreign table modification

When WCOs are present, disable direct foreign table modification.

If the user modifies a view that has CHECK OPTIONs and this gets
translated into a modification to an underlying relation which happens
to be a foreign table, the check options should be enforced. In the
normal code path, that was happening properly, but it was not working
properly for "direct" modification because the whole operation gets
pushed to the remote side in that case and we never have an option to
enforce the constraint against individual tuples. Fix by disabling
direct modification when there is a need to enforce CHECK OPTIONs.

Etsuro Fujita, reviewed by Kyotaro Horiguchi and by me.

Discussion: http://postgr.es/m/f8a48f54-6f02-9c8a-5250-9791603171ee@lab.ntt.co.jp

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/971faefc240bf214076e529773c7ca690c5e053d

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 60 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 24 +++++++++++
doc/src/sgml/postgres-fdw.sgml | 6 ++-
src/backend/optimizer/plan/createplan.c | 7 ++-
4 files changed, 93 insertions(+), 4 deletions(-)