add HPN-SSH option to openssh

This adds an option to include the High Performance SSH/SCP (HPN-SSH) patch from http://www.psc.edu/index.php/hpn-ssh.

For the last years, this was kept at https://github.com/ccosby/homebrew-ninja, but it might be more visible in this repo.

We understand this it can not included in the main openssh.rb, as hpn-ssh patches only become available with a delay. I hope that this repo is the right place.