Fix bugs in bindresvport_sa() changes

In svctcp_create() and svcudp_bufcreate(), set sa->sa_len on platforms
where that field exists, so that a subsequent call to socklen() will
return the correct result.

To make the code more self-evidently correct, zero the entire struct
sockaddr_storage object, using the memset(&ss, 0, sizeof(ss)) idiom.

ticket: 7935