typedef int (*SilcBufferFormatFunc)(SilcBuffer buffer,
                                        void *value,
                                        void *context);

    Formatting function callback given with SILC_STR_FUNC type.  The
    `buffer' is the buffer being formatted at the location where the
    SILC_STR_FUNC was placed in formatting.  The function should call
    silc_buffer_enlarge before it adds the data to the buffer to make
    sure that it has enough space.  The buffer->head points to the
    start of the buffer and silc_buffer_headlen() gives the length
    of the currently formatted data area.  It is also possible to use
    silc_buffer_format with `buffer' which will enlarge the buffer if
    needed.

    The `value' is the value given to SILC_STR_FUNC that is to be formatted
    into the buffer.  It may be NULL if the function is not formatting
    new data into the buffer.  The `context' is caller specific context.
    Returns -1 on error and length of the formatted value otherwise, and
    0 if nothing was formatted.