Fix escaping double-quotes in SQL identifiers. (#434)

* Fix escaping double-quotes in SQL identifiers.

When double-quoting SQL identifiers, any double-quote that is part of the
name should be doubled. Now, rather than implementing that ourselves yet
again in the C code, use format('%I', relname) as relname in the SQL
queries.

* Fix bugs.

* Update test files, the transform output has changed.