void silc_async_abort(SilcAsyncOperation op,
                          SilcAsyncOperationAbort abort_cb, void *context);

    This function is used by upper layer that received SilcAsyncOperation
    context from an asynchronous function, to abort the asynchronous
    operation.  The `op' becomes invalid after this function returns.
    It is also guaranteed (assuming the use of this API is implemented
    correctly) that some other completion callback is not called after
    the operation was aborted.  However, if the caller wants to receive
    a callback when aborting the caller may specify the `abort_cb' and
    `context' which will be called after the operation is aborted, but
    before the `op' becomes invalid.  The `abort_cb' is called immediately
    inside this function.