U2F support in OpenSSH HEAD

[Jordan J]

> That's possible already: at keygen time, the default is to require
> user presence for signatures but you can overide this by passing the
> "-x 0" flag. This is currently undocumented, and I'll hopefully soon
> get around to documenting it and making it accept mnemonic string
> instead of raw U2F flags.

I can make a quick pull request to update it to be a mnemonic if
you're interested. I'm looking around and testing the U2F support at
the moment anyway.