TCP RFC 6298 RTO handling

Versus the LoC, this is a high value change that improves TCP
performance in common [, tail] loss situations.

https://www.netbsd.org/~kbowling/tcp-rfc6298.patch

Even on high latency links, 1s is a good long while and a spurious RTO
may be profitable since links are generally less bandwidth constrained
while latency is what it is if you are dealing with tail loss in a
buffering scheme.

Regards,
Kevin Bowling