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/Client.cpp

@@ -10,9 +10,9 @@
 
 #include "Client.h"
 
-Client::Client(int id, CTCPSocket* socket, int ptm_fd)
+Client::Client(int id, ObjectPtr<CTCPSocket> socket, int ptm_fd)
     : m_id(id)
-    , m_socket(socket)
+    , m_socket(move(socket))
     , m_ptm_fd(ptm_fd)
     , m_ptm_notifier(CNotifier::create(ptm_fd, CNotifier::Read))
 {

Servers/TelnetServer/Client.h

@@ -11,15 +11,15 @@
 
 class Client : public RefCounted<Client> {
 public:
-    static NonnullRefPtr<Client> create(int id, CTCPSocket* socket, int ptm_fd)
+    static NonnullRefPtr<Client> create(int id, ObjectPtr<CTCPSocket> socket, int ptm_fd)
     {
-        return adopt(*new Client(id, socket, ptm_fd));
+        return adopt(*new Client(id, move(socket), ptm_fd));
     }
 
     Function<void()> on_exit;
 
 protected:
-    Client(int id, CTCPSocket* socket, int ptm_fd);
+    Client(int id, ObjectPtr<CTCPSocket> socket, int ptm_fd);
 
     void drain_socket();
     void drain_pty();
@@ -35,7 +35,7 @@ private:
     // client id
     int m_id { 0 };
     // client resources
-    CTCPSocket* m_socket { nullptr };
+    ObjectPtr<CTCPSocket> m_socket;
     Parser m_parser;
     // pty resources
     int m_ptm_fd { -1 };

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;