After 7e268107 started seeing warnings like
------------------
cdbgroup.c:1478:68: warning: expression which evaluates to zero treated as a null pointer constant of type 'List *' (aka 'struct List *') [-Wnon-literal-null-conversion]
                result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, false);
                                                                                 ^~~~~
------------------

A Motion node often needs to "merge" the incoming streams, to preserve the
overall sort order. Instead of carrying sort order information throughout
the later stages of planning, in the Flow struct, pass it as argument
directly to make_motion() and other functions, where a Motion node is
created. This simplifies things.

To make that work, we can no longer rely on apply_motion() to add the final
Motion on top of the plan, when the (sub-)query contains an ORDER BY. That's
because we no longer have that information available at apply_motion(). Add
the Motion node in grouping_planner() instead, where we still have that
information, as a path key.

When I started to work on this, this also fixed a bug, where the sortColIdx
of plan flow node may refer to wrong resno. A test case for that is
included. However, that case was since fixed by other coincidental changes
to partition elimination, so now this is just refactoring.

This bumps the copyright years to the appropriate years after not
having been updated for some time.  Also reformats existing code
headers to match the upstream style to ensure consistency.

There is no way we to reach PSETOP_NONE with the current coding, but
reading the code doesn't make that clear. Rewrite to make the intent
clearer.

Per gripe from Coverity.