ssh, HPN extension and TCP auto-tuning

[mlelstv%serpens.de@localhost (Michael van Elst)]

In https://mail-index.netbsd.org/current-users/2017/09/20/msg032361.html
there was a discussion about effectiveness of the "High Performance
Networking" patch to OpenSSH that we keep in our tree.

Details about the HPN patch can be found at:
https://www.psc.edu/hpn-ssh-home/hpn-ssh-faq/

This led finally to the decision to disable the HPN mode by default
by setting the Option HPNDisabled to yes in /etc/ssh/sshd_config
for netbsd-10.

I have now committed part of the latest version of the HPN changes
that handle TCP window auto-scaling, which avoids the issue we saw
in the past, at least in the network environments I can test.

I'd like to see tests from other people that may help with the decision
to re-enable it, or to abandon the HPN changes.