pgsql: Add locking clause for SB views for update/delete

Add locking clause for SB views for update/delete

In expand_security_qual(), we were handling locking correctly when a
PlanRowMark existed, but not when we were working with the target
relation (which doesn't have any PlanRowMarks, but the subquery created
for the security barrier quals still needs to lock the rows under it).

Noted by Etsuro Fujita when working with the Postgres FDW, which wasn't
properly issuing a SELECT ... FOR UPDATE to the remote side under a
DELETE.

Back-patch to 9.4 where updatable security barrier views were
introduced.

Per discussion with Etsuro and Dean Rasheed.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f16270aded0af5f3b19719bee3f67c8cc19b20aa

Modified Files
--------------
src/backend/optimizer/prep/prepsecurity.c | 35 +++-
src/test/regress/expected/updatable_views.out | 264 +++++++++++++------------
2 files changed, 170 insertions(+), 129 deletions(-)