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;
+};
+
+}

Userland/Libraries/LibC/bits/stdio_file_implementation.h

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Array.h>
+#include <AK/IntrusiveList.h>
 #include <AK/Types.h>
 #include <LibC/bits/FILE.h>
 #include <LibC/bits/pthread_integration.h>
@@ -127,6 +128,10 @@ private:
     pid_t m_popen_child { -1 };
     Buffer m_buffer;
     __pthread_mutex_t m_mutex;
+    IntrusiveListNode<FILE> m_list_node;
+
+public:
+    using List = IntrusiveList<&FILE::m_list_node>;
 };
 
 class ScopedFileLock {