pgsql: Fix oversight in recent changes to enable the 'physical tlist'

Log Message:
-----------
Fix oversight in recent changes to enable the 'physical tlist'
optimization for subquery and function scan nodes: we can't just do it
unconditionally, we still have to check whether there is any need for
a whole-row Var.  I had been thinking that these node types couldn't
have any system columns, which is true, but that loop is also checking
for attno zero, ie, whole-row Var.  Fix comment to not be so misleading.
Per test case from Richard Huxton.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        createplan.c (r1.201 -> r1.202)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/createplan.c.diff?r1=1.201&r2=1.202)