pgsql: RLS: Keep deny policy when only restrictive exist

RLS: Keep deny policy when only restrictive exist

Only remove the default deny policy when a permissive policy exists
(either from the hook or defined by the user).  If only restrictive
policies exist then no rows will be visible, as restrictive policies
shouldn't make rows visible.  To address this requirement, a single
"USING (true)" permissive policy can be created.

Update the test_rls_hooks regression tests to create the necessary
"USING (true)" permissive policy.

Back-patch to 9.5 where RLS was added.

Per discussion with Dean.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/8f439658524d4a3566682ff9e25d4791c5498e53

Modified Files
--------------
src/backend/rewrite/rowsecurity.c                        |   14 ++++++++++----
.../modules/test_rls_hooks/expected/test_rls_hooks.out   |    7 +++++++
src/test/modules/test_rls_hooks/sql/test_rls_hooks.sql   |    8 ++++++++
src/test/modules/test_rls_hooks/test_rls_hooks.c         |    5 +++++
4 files changed, 30 insertions(+), 4 deletions(-)