Yubikey for SSH login

• Upgrade openssh-server OpenSSH_7.6p1, to OpenSSH_8.0p1: https://gist.github.com/reanim8ed/1e76875dae00a57a7d5616636ba39f31

• yubikey-agent - a seamless ssh-agent for macOS and linux for YubiKeys: https://github.com/FiloSottile/yubikey-agent

• Yubikey cheatsheet: https://blog.programster.org/yubikey-cheatsheet#generate-public-rsa-certificate