Be a little more careful with the shift computations in QT2QTN and

makeTSQuerySign.  The first of these is a live bug, on some platforms,
as per bug #5590 from John Regehr.  However the consequences seem limited
because of the relatively narrow scope of use of QTNode.sign.  The shift in
makeTSQuerySign is actually safe because TSQS_SIGLEN is unsigned, but it
seems like a good idea to insert an explicit cast rather than depend on that.