bug#26696: openssh: root 'without-password & password-authentication #f both breaks service

[Christopher Allan Webber <cwebber@dustycloud.org>]

I wanted to permit root logins but only permit public key authentication
in my openssh configuration.  This was my original assumption of how to
do it:

  (service openssh-service-type
          (openssh-configuration
           (permit-root-login 'without-password)
           (password-authentication? #f)))

However, for whatever reason, openssh fails to start with this
combination.  However, it turns out this is redundant, since the
configuration is already only permitting with public key authentication.

  (service openssh-service-type
          (openssh-configuration
           (permit-root-login #t)
           (password-authentication? #f)))

This route is sufficient.

However maybe we should prevent people from accidentally causing openssh
to not start.  Here's a suggested route... though I haven't tested it:

#+BEGIN_SRC diff
diff --git a/gnu/services/ssh.scm b/gnu/services/ssh.scm
index 9917c311c..f1f2ab3dc 100644
--- a/gnu/services/ssh.scm
+++ b/gnu/services/ssh.scm
@@ -342,7 +342,13 @@ The other options should be self-descriptive."
                    #$(match (openssh-configuration-permit-root-login config)
                        (#t "yes")
                        (#f "no")
-                       ('without-password "without-password")))
+                       ('without-password
+                        ;; If we've already disabled password-authentication, this
+                        ;; is redundant, and even stops the openssh server from
+                        ;; starting up
+                        (if (openssh-configuration-password-authentication? config)
+                            "without-password"
+                            "yes"))))
            (format port "PermitEmptyPasswords ~a\n"
                    #$(if (openssh-configuration-allow-empty-passwords? config)
                          "yes" "no"))
#+END_SRC