Kernel: Rework TCP window size reporting

We now report actually available window size when sending packets. If
the available window size grows significantly we send an ACK to reflect
this to the remote.

kernel/include/kernel/Networking/TCPSocket.h

@@ -167,6 +167,8 @@ namespace Kernel
 		State m_next_state		{ State::Closed };
 		uint8_t m_next_flags	{ 0 };
 
+		size_t m_last_sent_window_size { 0 };
+
 		Thread* m_thread { nullptr };
 
 		// TODO: actually support these