Function knot_edns_opt_cookie_write() returns number of written bytes.

The [in/out] uint16_t *data_len has been removed to comply with the rest of the cookie code (where such arguments have already been removed).

The function now returns 0 on any error. This is not a big deal as the return value was only in resolver checked against KNOT_EOK.

I don't think that there would be a need for more elaborate error codes/values in this particular function.