Add CloseWrite() method to rwcConn in cnet/conn_rwc.go to support proper TCP connection closure in SOCKS5 mode.

[threatexpert]

I encountered an issue while using Chisel in SOCKS5 mode, where the following connection flow caused unexpected behavior:

tcpclient ---use-socks5--> chisel-client --> chisel-server --> tcpserver
When the tcpserver actively closes the TCP connection, the chisel-server is notified correctly. However, the chisel-client does not receive the closure notification, which I believe is a bug.
If the tcpclient actively closes the TCP connection, the chisel-server successfully closes the connection with tcpserver, and everything works as expected.
Root Cause: Upon investigation, I found that when a channel object is passed through the go-socks5 process, the conversion to NewRWCConn(stream) prevents the CloseWrite method from being called on the connection.

In go-socks5/request.go, the following code is responsible for handling the connection closure:

type closeWriter interface {
    CloseWrite() error
}

// proxy is used to shuffle data from src to destination, and sends errors down a dedicated channel
func proxy(dst io.Writer, src io.Reader, errCh chan error) {
    _, err := io.Copy(dst, src)
    if tcpConn, ok := dst.(closeWriter); ok {
        tcpConn.CloseWrite()
    }
    errCh <- err
}

Here, the dst object needs to implement the closeWriter interface. When tcpserver actively closes the TCP connection, the chisel-server attempts to close the channel. However, since the object net.Conn from NewRWCConn does not implement CloseWrite, the connection closure does not propagate properly.

Solution: I believe adding a CloseWrite() method to cnet/conn_rwc.go would resolve this issue. The method would attempt to call CloseWrite on the underlying ReadWriteCloser object, if it supports it.

The proposed method is:

type closeWriter interface {
    CloseWrite() error
}

func (c *rwcConn) CloseWrite() error {
    if cw, ok := c.ReadWriteCloser.(closeWriter); ok {
        return cw.CloseWrite()
    }
    return nil
}

This change ensures that the rwcConn type can correctly close connections when necessary, especially when the connection involves a channel object that supports the CloseWrite() method. This will fix the issue where chisel-client fails to receive the connection closure notification when tcpserver closes the connection.