Fix for broken JDBC's getColumn() (take 2)

Hi,

Per the recent discussion there's been some code changes in JDBC's
DatabaseMetaData.getColumn(). I proposed a patch that would change the
number of queries to find out all columns in a table from 2 * N + 1 to 1 (N
being the number of columns reported) by using some outer joins. I also
fixed the fact that getColumns() only returned columns that had a default
defined. OTOH, I did not use to change the code required for obtaining a
column's remarks (by using col_description() for 7.2 and requested by Tom
Lane).

Finally, I have found a way to get all the column details in a single query
*and* use col_description() for 7.2 servers. A patch is attached. It
overrules René Pijlman's fix for this that was committed just today, but
still used N + 1 queries (sorry René ;-) )

I also fixed the return values for TABLE_CAT and TABLE_SCHEM from "" to
null, to be more standard compliant (and requested in René's mail found at
http://fts.postgresql.org/db/mw/msg.html?mid=1034253).

As always, the JDBC1 version has not been tested as I have no JDK 1.1
available.

Please review,

Jeroen