Minor preparatory refactoring for UPDATE row movement.

Minor preparatory refactoring for UPDATE row movement.

Generalize is_partition_attr to has_partition_attrs and make it
accessible from outside tablecmds.c.  Change map_partition_varattnos
to clarify that it can be used for mapping between any two relations
in a partitioning hierarchy, not just parent -> child.

Amit Khandekar, reviewed by Amit Langote, David Rowley, and me.
Some comment changes by me.

Discussion: http://postgr.es/m/CAJ3gD9fWfxgKC+PfJZF3hkgAcNOy-LpfPxVYitDEXKHjeieWQQ@mail.gmail.com