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.

Servers/TelnetServer/main.cpp

@@ -107,7 +107,7 @@ int main(int argc, char** argv)
     server.on_ready_to_accept = [&next_id, &clients, &server] {
         int id = next_id++;
 
-        auto* client_socket = server.accept();
+        auto client_socket = server.accept();
         if (!client_socket) {
             perror("accept");
             return;