LibCore: Convert CTCPSocket to ObjectPtr, add construct() helper

The C_OBJECT macro now also inserts a static construct(...) helper into
the class. Now we can make the constructor(s) private and instead call:

    auto socket = CTCPSocket::construct(arguments);

construct() returns an ObjectPtr<T>, which we'll later switch to being
a NonnullRefPtr<T>, once everything else in in place for ref-counting.

Libraries/LibCore/CHttpJob.cpp

@@ -116,7 +116,7 @@ void CHttpJob::finish_up()
 void CHttpJob::start()
 {
     ASSERT(!m_socket);
-    m_socket = new CTCPSocket(this);
+    m_socket = CTCPSocket::construct(this);
     m_socket->on_connected = [this] {
         dbg() << "CHttpJob: on_connected callback";
         on_socket_connected();