Put a tag-length-value encoded attribute.

def TLV_PUT(attrs, attrNum, format, value):
    """ Put a tag-length-value encoded attribute. """
    attrView = attrs[attrNum]
    if format == 's':
        format = str(attrView.len) + format
    struct.pack_into(format, attrView.buf, attrView.offset, value)