pgsql: Fix misbehavior of DROP OWNED BY with duplicate polroles entries

Fix misbehavior of DROP OWNED BY with duplicate polroles entries.

Ordinarily, a pg_policy.polroles array wouldn't list the same role
more than once; but CREATE POLICY does not prevent that.  If we
perform DROP OWNED BY on a role that is listed more than once,
RemoveRoleFromObjectPolicy either suffered an assertion failure
or encountered a tuple-updated-by-self error.  Rewrite it to cope
correctly with duplicate entries, and add a CommandCounterIncrement
call to prevent the other problem.

Per discussion, there's other cleanup that ought to happen here,
but this seems like the minimum essential fix.

Per bug #17062 from Alexander Lakhin.  It's been broken all along,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/17062-11f471ae3199ca23@postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0b29b41e5b9614397ba839552f35b4485ca49e0f

Modified Files
--------------
src/backend/commands/policy.c             | 50 +++++++++++++++++--------------
src/test/regress/expected/rowsecurity.out |  8 +++++
src/test/regress/sql/rowsecurity.sql      |  9 ++++++
3 files changed, 44 insertions(+), 23 deletions(-)