Kernel: Send ACK/FIN in response to FIN packets on active connections

This is to work around our lack of a shutdown() implementation.

Kernel/Net/NetworkTask.cpp

@@ -527,8 +527,12 @@ void handle_tcp(const IPv4Packet& ipv4_packet)
  socket->did_receive(ipv4_packet.source(), tcp_packet.source_port(), KBuffer::copy(&ipv4_packet, sizeof(IPv4Packet) + ipv4_packet.payload_size()));
 
  socket->set_ack_number(tcp_packet.sequence_number() + payload_size + 1);
- socket->send_tcp_packet(TCPFlags::ACK);
- socket->set_state(TCPSocket::State::CloseWait);
+ // TODO: We should only send a FIN packet out once we're shutting
+ // down our side of the socket, so we should change this back to
+ // just being an ACK and a transition to CloseWait once we have a
+ // shutdown() implementation.
+ socket->send_tcp_packet(TCPFlags::FIN | TCPFlags::ACK);
+ socket->set_state(TCPSocket::State::Closing);
  socket->set_connected(false);
  return;
  }