Merry Christmas, here’s an incredibly involved bugfix

There’s been a rare segfault present in DragonFly for quite some time.  It’s been difficult to reproduce, and the 2.12 release due some months ago was held up specifically to fix it.  Matthew Dillon was, after many days (months?) of work, able to replicate it reliably and eventually find a way around what appears to be a new AMD-specific bug.  Read his very detailed explanation of what he did to get to this point.