Added simplest device mount implementation (includes NullDevice and RandomDevice)

libraries/nacl-mounts/dev/RandomDevice.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/RandomDevice.h"
+
+int RandomDevice::Read(off_t offset, void *buf, size_t count) {
+  int nread;
+  get_random_bytes_(buf, count, &nread);
+  return nread;
+}
+
+int RandomDevice::Write(off_t offset, const void *buf, size_t count) {
+  return EPERM;
+}
+