allow_system_table_mods and DROP RULE

As a curious omission, DROP RULE does not check allow_system_table_mods. 
  Creating and renaming a rule does, and also creating, renaming, and 
dropping a trigger does.  The impact of this is probably nil in 
practice, but for consistency we should probably add that.  The patch is 
pretty simple.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services