Fix Var handling for security barrier views

 Fix Var handling for security barrier views
 
 In some cases, not all Vars were being correctly marked as having been
 modified for updatable security barrier views, which resulted in invalid
 plans (eg: when security barrier views were created over top of
 inheiritance structures).
 
 In passing, be sure to update both varattno and varonattno, as _equalVar
 won't consider the Vars identical otherwise.  This isn't known to cause
 any issues with updatable security barrier views, but was noticed as
 missing while working on RLS and makes sense to get fixed.
 
 Back-patch to 9.4 where updatable security barrier views were
 introduced.