Add a C++ helper class for working with shared buffers.

This is a bit more comfortable than passing the shared buffer ID manually
everywhere and keeping track of size etc.

Kernel/MemoryManager.h

@@ -203,6 +203,8 @@ public:
 
     const Bitmap& cow_map() const { return m_cow_map; }
 
+    void set_writable(bool b) { m_writable = b; }
+
 private:
     RetainPtr<PageDirectory> m_page_directory;
     LinearAddress m_laddr;