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/LibELF/DynamicObject.h

@@ -277,6 +277,8 @@ public:
 
     bool elf_is_dynamic() const { return m_is_elf_dynamic; }
 
+    void* symbol_for_name(const StringView& name);
+
 private:
     explicit DynamicObject(const String& filename, VirtualAddress base_address, VirtualAddress dynamic_section_address);