[$] Preventing kernel-stack leaks

The kernel stack is a small, frequently reused region of memory in eachthread's address space. That reuse allows for efficient memory use andgood performance as a result of cache locality, but it also presents aproblem: data left on the stack can also end up being reused in ways thatwere not intended. The PaX patch set contains a mechanism designed toclear that data from the stack and prevent leaks, but an attempt to mergethat code into the kernel has run into a snag.