some platforms are very picky about the third argument passed to bind().

some platforms are very picky about the third argument passed to bind().
and would complain if sa.family is AF_INET and the third argument is not exactly the size of a sockaddr_in.

We used to pass a union containing both a sockaddr_in and a sockaddr_in6  which would mean that on those platforms bind() would fail since the passed structure for AF_INET would be too big.

Thus we need to set and pass the appropriate size to bind. At the same time for thos eplatforms we can also set sin[6]_size to the expected size.
(bind() on those platforms were isurprisingly perfectly ok with sin_len was "too big")