Added simplest device mount implementation (includes NullDevice and RandomDevice)

libraries/nacl-mounts/dev/NullDevice.cc

+/*
+ * Copyright (c) 2012 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "../dev/NullDevice.h"
+#include <cstring>
+
+int NullDevice::Read(off_t offset, void *buf, size_t count) {
+  if (count < 0)
+    return -1;
+  memset(buf, 0, count);
+  return count;
+}
+int Write(off_t offset, const void *buf, size_t count, size_t *nwrote) {
+  return count;
+}
+