Revert "Convert contrib/seg's bool-returning SQL functions to V1 call convention."

Revert "Convert contrib/seg's bool-returning SQL functions to V1 call convention."

This reverts commit b1dd2f86ce7d43f23f6aae307bb22de826849e7d.
That turns out to have been based on a faulty diagnosis of why the
VS2015 build was misbehaving.  Instead, we need to fix DatumGetBool().