pgsql: Create a pg_shdepend entry for each role in TO clause of policie

Create a pg_shdepend entry for each role in TO clause of policies.

CreatePolicy() and AlterPolicy() omit to create a pg_shdepend entry for
each role in the TO clause. Fix this by creating a new shared dependency
type called SHARED_DEPENDENCY_POLICY and assigning it to each role.

Reported by Noah Misch. Patch by me, reviewed by Alvaro Herrera.
Back-patch to 9.5 where RLS was introduced.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/992c9d345f6607c5b2cab2787f7cf72fba96673d

Modified Files
--------------
doc/src/sgml/catalogs.sgml                |   10 ++++
src/backend/catalog/pg_shdepend.c         |    2 +
src/backend/commands/policy.c             |   87 ++++++++++++++++++++---------
src/include/catalog/dependency.h          |    5 ++
src/test/regress/expected/rowsecurity.out |   55 ++++++++++++++++++
src/test/regress/sql/rowsecurity.sql      |   44 +++++++++++++++
6 files changed, 177 insertions(+), 26 deletions(-)