#define SILC_STR_OFFSET() ...

    Offset in buffer.  This can be used in formatting and unformatting to
    move the data pointer of the buffer either forwards (positive offset)
    or backwards (negative offset).  It can be used to for example skip
    some types during unformatting.

    Example:

    ..., SILC_STR_OFFSET(5), ...
    ..., SILC_STR_OFFSET(-3), ...

    Moves the data pointer at the point of the offset either forward
    or backward and then moves to the next type.  Multiple SILC_STR_OFFSETs
    can be used in formatting and unformatting at the same time.