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.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#define RTLD_DEFAULT 0
+#define RTLD_LAZY 2
+#define RTLD_NOW 4
+#define RTLD_GLOBAL 8
+#define RTLD_LOCAL 16
+
+int dlclose(void*);
+char* dlerror();
+void* dlopen(const char*, int);
+void* dlsym(void*, const char*);
+
+__END_DECLS