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/CTCPServer.h

@@ -15,7 +15,7 @@ public:
     bool is_listening() const { return m_listening; }
     bool listen(const IPv4Address& address, u16 port);
 
-    CTCPSocket* accept();
+    ObjectPtr<CTCPSocket> accept();
 
     Function<void()> on_ready_to_accept;