LibC: Implement fflush(nullptr)

This caused all open file streams to be flushed.

This commit also changes `FILE::create` to handle buffer allocation
failure gracefully.

Userland/Libraries/LibC/bits/mutex_locker.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <pthread.h>
+
+// We don't want to bring LibThreading headers into LibC, so we use plain
+// pthread mutexes and this RAII guard.
+namespace LibC {
+
+class [[nodiscard]] MutexLocker {
+public:
+    explicit MutexLocker(pthread_mutex_t& mutex)
+        : m_mutex(mutex)
+    {
+        lock();
+    }
+
+    ~MutexLocker()
+    {
+        unlock();
+    }
+
+    void lock() { pthread_mutex_lock(&m_mutex); }
+    void unlock() { pthread_mutex_unlock(&m_mutex); }
+
+private:
+    pthread_mutex_t& m_mutex;
+};
+
+}