pgsql: Perform RLS subquery checks as the right user when going via av

Perform RLS subquery checks as the right user when going via a view.

When accessing a table with RLS via a view, the RLS checks are
performed as the view owner. However, the code neglected to propagate
that to any subqueries in the RLS checks. Fix that by calling
setRuleCheckAsUser() for all RLS policy quals and withCheckOption
checks for RTEs with RLS.

Back-patch to 9.5 where RLS was added.

Per bug #15708 from daurnimator.

Discussion: https://postgr.es/m/15708-d65cab2ce9b1717a@postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1b9a2f458b36747db07aff6631ade56c24cfda03

Modified Files
--------------
src/backend/rewrite/rowsecurity.c         |  8 ++++++++
src/test/regress/expected/rowsecurity.out | 27 +++++++++++++++++++++++++++
src/test/regress/sql/rowsecurity.sql      | 26 ++++++++++++++++++++++++++
3 files changed, 61 insertions(+)