Different error messages executing CREATE TABLE or ALTER TABLE to create a column "xmin"

I took my first stab at hacking the sources to fix the bug reported here:

http://archives.postgresql.org/pgsql-bugs/2012-01/msg00152.php

It's such a simple patch but it took me several hours with Google and IRC
and I'm sure I did many things wrong.  It seems to work as advertised,
though, so I'm submitting it.

I suppose since I have now submitted a patch, it is my moral obligation to
review at least one.  I'm not sure how helpful I'll be, but I'll go bite
the bullet and sign myself up anyway.