TelnetServer: Implement basic telnet server

Fixes #407

Depends on #530 to run reliably.

Servers/TelnetServer/Client.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <AK/Types.h>
+#include <LibCore/CNotifier.h>
+#include <LibCore/CTCPSocket.h>
+
+#include "Command.h"
+#include "Parser.h"
+
+class Client : public RefCounted<Client> {
+public:
+    static NonnullRefPtr<Client> create(int id, CTCPSocket* socket, int ptm_fd)
+    {
+        return adopt(*new Client(id, socket, ptm_fd));
+    }
+
+    Function<void()> on_exit;
+
+protected:
+    Client(int id, CTCPSocket* socket, int ptm_fd);
+
+    void drain_socket();
+    void drain_pty();
+    void handle_data(const StringView&);
+    void handle_command(const Command& command);
+    void handle_error();
+    void send_data(StringView str);
+    void send_command(Command command);
+    void send_commands(Vector<Command> commands);
+    void quit();
+
+private:
+    // client id
+    int m_id { 0 };
+    // client resources
+    CTCPSocket* m_socket { nullptr };
+    Parser m_parser;
+    // pty resources
+    int m_ptm_fd { -1 };
+    CNotifier m_ptm_notifier;
+};