Subject: Add an internal SFTP server to the OpenSSH service.

From: Clément Lassieur <[email-masked]>
Date: Sun, 19 Mar 2017 17:32:13 +0100

The goal of this patchset is to add a 'subsystems' field to
<openssh-configuration>, whose default value would be an internal SFTP
server.

I also added a test for this SFTP server, as asked by Ludovic.  For this
test, I needed to abstract the session connection and authentication
stuff: I took inspiration from the Guile-SSH tests.

Previous thread was:
http://lists.gnu.org/archive/html/guix-devel/2017-03/msg00461.html.

Comments are welcome :)
Clément