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