pgsql: Disallow dropping rules on system tables by default

Disallow dropping rules on system tables by default

This was previously not covered by allow_system_table_mods, but now it
is. The impact in practice is probably low, but this makes it
consistent with most other DDL commands.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: /messages/by-id/ee9df1af-c0d8-7c82-5be7-39ce4e3b0a9d@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/df7fe9e2d707da69a4437fb6f9e695c070882160

Modified Files
--------------
src/backend/rewrite/rewriteRemove.c | 8 ++++++++
src/test/modules/unsafe_tests/expected/alter_system_table.out | 11 ++++++++++-
src/test/modules/unsafe_tests/sql/alter_system_table.sql | 10 +++++++++-
3 files changed, 27 insertions(+), 2 deletions(-)