pgsql: Disallow converting a table to a view if row security is present

Disallow converting a table to a view if row security is present.

When DefineQueryRewrite() is about to convert a table to a view, it checks
the table for features unavailable to views.  For example, it rejects tables
having triggers.  It omits to reject tables having relrowsecurity or a
pg_policy record. Fix that. To faciliate the repair, invent
relation_has_policies() which indicates the presence of policies on a
relation even when row security is disabled for that relation.

Reported by Noah Misch. Patch by me, review by Stephen Frost. Back-patch
to 9.5 where RLS was introduced.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/d824e2800f66f6180189d973c720611855c6f619

Modified Files
--------------
src/backend/commands/policy.c             |   29 +++++++++++++++++++++++++++++
src/backend/rewrite/rewriteDefine.c       |   24 +++++++++++++++++++-----
src/include/commands/policy.h             |    1 +
src/test/regress/expected/rowsecurity.out |   23 +++++++++++++++++++++++
src/test/regress/sql/rowsecurity.sql      |   25 +++++++++++++++++++++++++
5 files changed, 97 insertions(+), 5 deletions(-)