cygport upload: patch for openssh 6.8p1

[Andrew Schulman]

> > Any thoughts on a better regex or on keeping compatibility with other
> > systems?
> 
> Right, OK.  See the attached revised patch, which uses
> 
>   [0-9a-f]{2}(:[0-9a-f]{2}){15}|SHA256:.{44}
> 
> to detect the key fingerprint.  The left side is the same as now, for pre-6.8
> systems, which use MD5 without a label.  The right side is for version 6.8 and
> later, where the default is SHA256 with the label 'SHA256:' prepended.  So this
> should cover all cases.

A more precise regex is

    [0-9a-f]{2}(:[0-9a-f]{2}){15}|SHA256:[A-Za-z0-9+/=]{43}

I've committed this change to my upload branch at
https://github.com/andrex-e-schulman/cygport.git.  

Also in that branch, I've added documentation of the fact that users will need
to connect to cygwin.com at least one time by sftp before they upload, in order
to store the host public key in their known_hosts file, as explained in
https://cygwin.com/ml/cygwin-apps/2015-03/msg00193.html .

Andrew