[$] Another attempt at speculative page-fault handling

While the best way to avoid performance problems associated with pagefaults is usually to avoid faulting altogether, that is not always anoption. Thus, it is important that the kernel handle page faults with aminimum of overhead. One particular pain point in current kernels comesabout in multi-threaded workloads that are all incurring faults in thesame address space. Speculative page-fault handling is an old idea forimproving the scalability of such workloads that may finally be approachinga point where it can be considered for inclusion.