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.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gunnar@beutner.name>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/String.h>
+#include <AK/Types.h>
+#include <LibDl/dlfcn.h>
+#include <LibDl/dlfcn_integration.h>
+#include <string.h>
+
+// FIXME: use thread_local and a String once TLS works
+__thread char* s_dlerror_text = NULL;
+__thread bool s_dlerror_retrieved = false;
+
+static void store_error(const String& error)
+{
+    free(s_dlerror_text);
+    s_dlerror_text = strdup(error.characters());
+    s_dlerror_retrieved = false;
+}
+
+int dlclose(void* handle)
+{
+    auto result = __dlclose(handle);
+    if (result.is_error()) {
+        store_error(result.error().text);
+        return -1;
+    }
+    return 0;
+}
+
+char* dlerror()
+{
+    if (s_dlerror_retrieved) {
+        free(s_dlerror_text);
+        s_dlerror_text = nullptr;
+    }
+    s_dlerror_retrieved = true;
+    return const_cast<char*>(s_dlerror_text);
+}
+
+void* dlopen(const char* file_name, int flags)
+{
+    auto result = __dlopen(file_name, flags);
+    if (result.is_error()) {
+        store_error(result.error().text);
+        return nullptr;
+    }
+    return result.value();
+}
+
+void* dlsym(void* handle, const char* symbol_name)
+{
+    auto result = __dlsym(handle, symbol_name);
+    if (result.is_error()) {
+        store_error(result.error().text);
+        return nullptr;
+    }
+    return result.value();
+}