Implement scandirat and scandirat64.

Bug: http://b/12612339
Change-Id: Id3b249a884fe08964b26a017ae9574961f0cb441
diff --git a/libc/private/ScopedReaddir.h b/libc/private/ScopedReaddir.h
index 84c1b93..3d77a40 100644
--- a/libc/private/ScopedReaddir.h
+++ b/libc/private/ScopedReaddir.h
@@ -23,8 +23,11 @@
 
 class ScopedReaddir {
  public:
-  ScopedReaddir(const char* path) {
-    dir_ = opendir(path);
+  ScopedReaddir(const char* path) : ScopedReaddir(opendir(path)) {
+  }
+
+  ScopedReaddir(DIR* dir) {
+    dir_ = dir;
   }
 
   ~ScopedReaddir() {