Give each task its own page directory.

This isn't finished but I'll commit as I go. We need to get to where context
switching only needs to change CR3 and everything's ready to go.

My basic idea is:
- The first 4 kB is off-limits. This catches null dereferences.
- Up to the 4 MB mark is identity-mapped and kernel-only.
- The rest is available to everyone!

While the first 4 MB is only available to the kernel, it's still mapped in
every process, for convenience when entering the kernel.

Kernel/MemoryManager.h

@@ -65,6 +65,8 @@ public:
     void registerZone(Zone&);
     void unregisterZone(Zone&);
 
+    void populatePageDirectory(Task&);
+
 private:
     MemoryManager();
     ~MemoryManager();
@@ -158,7 +160,7 @@ private:
         dword* m_pte;
     };
 
-    PageTableEntry ensurePTE(LinearAddress);
+    PageTableEntry ensurePTE(dword* pageDirectory, LinearAddress);
 
     dword* m_pageDirectory;
     dword* m_pageTableZero;