some more error location support

Here are three patches to add more detailed error location support to
some parts of CREATE TABLE (defaults, check constraints, partition
specifications) as well as the COPY command. Examples can be seen in
the regression test output.

The first two are low-hanging fruit, since all the information was
already collected and just needed to be passed through one last hop.
The third one is a bit more invasive; it adds location information to
the Value node, which is used in a variety of commands, so COPY is just
a start here.

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Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services