LibC+LibELF: Implement more fully-features dlfcn functionality

This implements more of the dlfcn functionality. Most notably:

* It's now possible to dlopen() libraries which were already
  loaded at program startup time. This does not cause those
  libraries to be loaded twice.
* Errors are reported via dlerror() rather than by crashing
  the program.
* Calls to the dl*() functions are thread-safe.

Userland/Libraries/LibDl/dlfcn_integration.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gunnar@beutner.name>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Result.h>
+#include <AK/String.h>
+
+struct DlErrorMessage {
+    DlErrorMessage(String&& other)
+        : text(move(other))
+    {
+    }
+
+    // The virtual destructor is required because we're passing this
+    // struct to the dynamic loader - whose operator delete differs
+    // from the one in libc.so
+    virtual ~DlErrorMessage() { }
+
+    String text;
+};
+
+typedef Result<void, DlErrorMessage> (*DlCloseFunction)(void*);
+typedef Result<void*, DlErrorMessage> (*DlOpenFunction)(const char*, int);
+typedef Result<void*, DlErrorMessage> (*DlSymFunction)(void*, const char*);
+
+extern "C" {
+extern DlCloseFunction __dlclose;
+extern DlOpenFunction __dlopen;
+extern DlSymFunction __dlsym;
+}