Change CREATE TABLE so that column default expressions coming from different

Change CREATE TABLE so that column default expressions coming from different
inheritance parent tables are compared using equal(), instead of doing
strcmp() on the nodeToString representation.  The old implementation was
always a tad cheesy, and it finally fails completely as of 8.4, now that the
node tree might contain syntax location information.  equal() knows it's
supposed to ignore those fields, but strcmp() hardly can.  Per recent
report from Scott Ribe.