LibIPC: Add IPC::take_over_accepted_client_from_system_server<Client>()

This is an encapsulation of the common work done by all of our
single-client IPC servers on startup:

    1. Create a Core::LocalSocket, taking over an accepted fd.
    2. Create an application-specific ClientConnection object,
       wrapping the socket.

It's not a huge change in terms of lines saved, but I do feel that it
improves expressiveness. :^)

Userland/Libraries/LibIPC/SingleServer.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibIPC/ClientConnection.h>
+
+namespace IPC {
+
+template<typename ClientConnectionType>
+ErrorOr<NonnullRefPtr<ClientConnectionType>> take_over_accepted_client_from_system_server()
+{
+    auto socket = TRY(Core::LocalSocket::take_over_accepted_socket_from_system_server());
+    return IPC::new_client_connection<ClientConnectionType>(move(socket));
+}
+
+}