The priority boosting mechanism has been broken for a very long time.
Let's remove it from the codebase and we can bring it back the day
someone feels like implementing it in a working way. :^)
This patch adds a new AnonymousFile class which is a File backed by
an AnonymousVMObject that can only be mmap'ed and nothing else, really.
I'm hoping that this can become a replacement for shbufs. :^)
