patch: enforce the requirements for scrollable resultsets

This patch (against CVS HEAD) adds enforcement of the requirement that
you're working with a scrollable resultset before calling some methods
(last(), absolute(), etc) of ResultSet. Without this patch, these
methods complete "normally" but can return incorrect data if the
resultset is backed by a cursor. It also adds tests for this behaviour,
and fixes a number of tests and one case in the driver itself that try
to use these methods with the wrong resultset type.

There are also a couple of minor other fixes in related areas that I
made along the way.

Details:

- Add AbstractJdbc2ResultSet.checkScrollable() that throws if the
resultset is not scrollable, call it from appropriate methods.
- Allow ResultSet.absolute(0) per the spec: it should position before
the start of the resultset, not throw an exception. Add a test for this
case.
- Check the ResultSet.setFetchDirection() parameter and throw an
exception if it's not one of the allowed values.
- Use rs.next() not rs.first() in AbstractJdbc2ResultSet.refreshRow(),
as the resultset in use is not scrollable.
- If a SQLException is caught in AbstractJdbc2ResultSet.refreshRow(),
rethrow it unchanged to preserve the stack trace.
- Fix multiple cases in ResultSetTest where we were expecting a
scrollable resultset but didn't ask for one.
- Add a testcase for throwing an exception when inappropriate methods
are called on TYPE_FORWARD_ONLY resultsets.

-O