Report initial ssthresh value as 0 while debugging.

Report initial ssthresh value as 0 while debugging.

The initial is actually UINT_MAX, but since this makes it hard to plot a
graph of it with autoscaling axes, show it as 0 instead. This removes the
hack where ssthresh is set to sndbuf.maxsize at the start of a connection,
which is wrong when the application overrides the send buffer size later.