SilcSFTP silc_sftp_client_start(SilcSFTPSendPacketCallback send_packet,
                                    void *send_context,
                                    SilcSFTPVersionCallback callback,
                                    void *context);

    Starts SFTP client and returns context to it.  The version callback
    indicated by the `callback' will be called after the SFTP session has
    been started and server has returned the version of the protocol.  The
    SFTP client context is returned in the callback too.  This returns the
    allocated SFTP client context or NULL on error.  Each socket connection
    should allocate their own SFTP client by calling this function.