CREATE TABLE LIKE x INCLUDING CONSTRAINTS

Fixed previous patch by calling change_varattnos_of_a_node() to fix up
constraint expressions in case attribute positions don't line up.

change_varattnos_of_a_node is in tablecmds.c for inherited tables so this
means making it extern. I have a feeling it probably ought to move to some
file of functions for manipulating Nodes but I couldn't really find an
appropriate place. At first I was going to put it in ruleutils.c but then it
seems the other functions in that file go in builtins.h which would be a
strange place for this I think.

So in the end I left the functions in tablecmds.[ch] at least until someone
more familiar with the source tree suggests another pair of files for them to
be defined in.