Use a varno not chosen at random for dummy variables in the top-level

Use a varno not chosen at random for dummy variables in the top-level
targetlist of a set-operation tree.  I'm not sure that this solution
will really stand the test of time --- perhaps we need to make a special
RTE for such vars to refer to.  But this quick hack fixes Brandon Craig
Rhodes' complaint of 10-Feb-02 about EXCEPT in CREATE RULE, while not
changing any behavior in the better-tested cases where leftmostRTI is
one anyway.