Another spin of the prototype.  This one avoids the espfix for anything
but #GP, and avoids save/restore/saving registers... one can wonder,
though, how much that actually matters in practice.

It still does redundant SWAPGS on the slow path.  I'm not sure I
personally care enough to optimize that, as it means some fairly
significant restructuring of some of the code paths.  Some of that
restructuring might actually be beneficial, but still...

	-hpa