SystemServer+LibCore: Implement socket takeover

SystemServer can now create sockets on behalf of services before spawning any
of them, and pass the open socket fd as fd 3. CLocalServer gains a method to
complete the takeover and listen on the passed fd.

This is not used by any services at the moment.

Libraries/LibCore/CLocalServer.h

@@ -10,6 +10,7 @@ class CLocalServer : public CObject {
 public:
     virtual ~CLocalServer() override;
 
+    bool take_over_from_system_server();
     bool is_listening() const { return m_listening; }
     bool listen(const String& address);
 
@@ -20,6 +21,8 @@ public:
 private:
     explicit CLocalServer(CObject* parent = nullptr);
 
+    void setup_notifier();
+
     int m_fd { -1 };
     bool m_listening { false };
     RefPtr<CNotifier> m_notifier;