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

    Unformatting function callback given with SILC_STR_FUNC type.  The
    `buffer' is the buffer being unformatted and is at the location where
    the SILC_STR_FUNC was placed in unformatting.  The function should
    check there is enough data in the `buffer' before trying to decode
    from it.

    If this function unformats anything from the buffer its value is to
    be returned to the `value' pointer.  The implementation should itself
    decide whether the unformatted value is allocated or not.  If this
    function does not unformat anything, nothing is returned to `value'

    The `context' is caller specific context.  Returns -1 on error, and
    length of the unformatted value otherwise, and 0 if nothing was
    unformatted.