Append and SubqueryScan nodes were not passing changed-parameter signals down

Append and SubqueryScan nodes were not passing changed-parameter signals down
to their children, leading to misbehavior if they had any children that paid
attention to chgParam (most plan node types don't).  Append's bug has been
there a long time, but nobody had noticed because it used to be difficult
to create a query where an Append would be used below the top level of a
plan; so there were never any parameters getting passed down.  SubqueryScan
is new in 7.1 ... and I'd modeled its behavior on Append :-(