typedef void (*SilcSFTPDataCallback)(SilcSFTP sftp,
                                         SilcSFTPStatus status,
                                         const unsigned char *data,
                                         SilcUInt32 data_len,
                                         void *context);

    Data callback is called when data packet is received from the server.
    This is called for example when application is reading a file from
    the server.  The `data' is the raw data of length of `data_len'.