Don't issue a BEGIN when running VACUUM in auto-commit mode.

Don't issue a BEGIN when running VACUUM in auto-commit mode.

Normally in auto-commit mode the driver begins a new transaction
implicitly at the first statement, by sending a BEGIN statement. However,
some commands, like VACUUM, cannot be run in a transaction block, and you
will get an error like "VACUUM cannot run inside a transaction block" from
the server. In UseServerSidePrepare=0 mode, the code looks at the first word
of the query to determine if the statement is one of the special ones, and
if so, didn't begin a new transaction even when auto-commit mode is
disabled. However, in UseServerSidePrepare=1 mode, when using SQLPrepare/
SQLExecute to run the VACUUM, that check was not made. Fix that.

There was one more related inconsistency between UseServerSidePrepare modes.
Without server-side-prepares, if you issued an explicit BEGIN in auto-commit
mode, the implicit BEGIN was ont sent. But without server-side prepares, it
was. It seems best to send the implicit BEGIN in both cases, because then
you get a warning from the backend about the second BEGIN. That's a good
thing, because a sane ODBC application should be using the ODBC function
SQLEndTran() for transaction control, not explicit BEGIN/COMMIT.

Also add a test case for executing VACUUM, with and without autocommit.