Fix dlerror(3).

Add unit tests for dlerror(3) in various situations. I think We're at least
as good as glibc now.

Also factor out the ScopedPthreadMutexLock and use it here too.

Bug: http://code.google.com/p/android/issues/detail?id=38398
Change-Id: I040938b4366ab836e3df46d1d8055b92f4ea6ed8
diff --git a/libc/private/ScopedPthreadMutexLocker.h b/libc/private/ScopedPthreadMutexLocker.h
new file mode 100644
index 0000000..06b8e37
--- /dev/null
+++ b/libc/private/ScopedPthreadMutexLocker.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef SCOPED_PTHREAD_MUTEX_LOCKER_H
+#define SCOPED_PTHREAD_MUTEX_LOCKER_H
+
+#include <pthread.h>
+
+class ScopedPthreadMutexLocker {
+ public:
+  explicit ScopedPthreadMutexLocker(pthread_mutex_t* mu) : mu_(mu) {
+    pthread_mutex_lock(mu_);
+  }
+
+  ~ScopedPthreadMutexLocker() {
+    pthread_mutex_unlock(mu_);
+  }
+
+ private:
+  pthread_mutex_t* mu_;
+
+  // Disallow copy and assignment.
+  ScopedPthreadMutexLocker(const ScopedPthreadMutexLocker&);
+  void operator=(const ScopedPthreadMutexLocker&);
+};
+
+#endif // SCOPED_PTHREAD_MUTEX_LOCKER_H