More work on per-process page directories. It basically works now!

I spent some time stuck on a problem where processes would clobber each
other's stacks. Took me a moment to figure out that their stacks
were allocated in the sub-4MB linear address range which is shared
between all processes. Oops!

Kernel/ProcFileSystem.cpp

@@ -61,6 +61,7 @@ ByteBuffer procfs$pid_vm(const Task& task)
 ByteBuffer procfs$pid_stack(Task& task)
 {
     InterruptDisabler disabler;
+    ASSERT(false);
     if (current != &task) {
         MM.unmapRegionsForTask(*current);
         MM.mapRegionsForTask(task);