services: openssh: Fix 'PrintLastLog' default behaviour.

* gnu/services/ssh.scm (openssh-config-file): Add 'print-last-log?' option.
(<openssh-configuration>)[print-last-log?]: Add it.
(openssh-activation): Touch /var/log/lastlog.
* doc/guix.texi (Networking Services): Document 'print-last-log?'.

Before that, the service did not work as expected because /var/log/lastlog did
not exist.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>

doc/guix.texi

@@ -9352,6 +9352,10 @@ Because PAM challenge response authentication usually serves an
 equivalent role to password authentication, you should disable either
 @code{challenge-response-authentication?} or
 @code{password-authentication?}.
+
+@item @code{print-last-log?} (default: @code{#t})
+Specifies whether @command{sshd} should print the date and time of the
+last user login when a user logs in interactively.
 @end table
 @end deftp

gnu/services/ssh.scm

@@ -279,6 +279,8 @@ (define-record-type* <openssh-configuration>
   (challenge-response-authentication? openssh-challenge-response-authentication?
                                       (default #f)) ;Boolean
   (use-pam?              openssh-configuration-use-pam?
+                         (default #t)) ;Boolean
+  (print-last-log?       openssh-configuration-print-last-log?
                          (default #t))) ;Boolean
 
 (define %openssh-accounts
@@ -298,6 +300,14 @@ (define (openssh-activation config)
       (mkdir-p "/etc/ssh")
       (mkdir-p (dirname #$(openssh-configuration-pid-file config)))
 
+      (define (touch file-name)
+        (call-with-output-file file-name (const #t)))
+
+      (let ((lastlog "/var/log/lastlog"))
+        (when #$(openssh-configuration-print-last-log? config)
+          (unless (file-exists? lastlog)
+            (touch lastlog))))
+
       ;; Generate missing host keys.
       (system* (string-append #$(openssh-configuration-openssh config)
                               "/bin/ssh-keygen") "-A")))
@@ -336,6 +346,9 @@ (define (openssh-config-file config)
          (format port "UsePAM ~a\n"
                  #$(if (openssh-configuration-use-pam? config)
                        "yes" "no"))
+         (format port "PrintLastLog ~a\n"
+                 #$(if (openssh-configuration-print-last-log? config)
+                       "yes" "no"))
          #t))))
 
 (define (openssh-shepherd-service config)