Win: EADDRNOTAVAIL when all the ports are in use

When several connections are opened to the same host on the same port
on a very short time-span, all the ports will be exhausted and the
error returned was ENOBUFS. I think this error is misleading as I
immediately thought it was related to memory exhaustion on socket
buffers. A google search showed that there are more people assuming the
same.

The error I would expect for this case would be EADDRNOTAVAIL. While it
is still not perfect, I believe it's a stronger indication of the
underlying problem.