services: opensmtd: Add back option to log to syslog.

The option for syslog logging was removed in 6036f1f305.

* gnu/services/mail.scm (opensmtpd-shepherd-service): When log-file is #f
daemonize and set the pid-file.

Change-Id: I356f4ccacc473def35708c114947788d485d058b
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -29879,6 +29879,7 @@ remote servers.  Run @command{man smtpd.conf} for more information.
 @item @code{log-file} (default: @code{"/var/log/mail.log"})
 The file location for the OpenSMTPD log file.  Logging occurs in the
 foreground via the Shepherd, i.e. OpenSMTPD does not detach.
+When set to @code{#f} OpenSMTPD will instead detach and log to the syslog.
 
 @item @code{setgid-commands?} (default: @code{#t})
 Make the following commands setgid to @code{smtpq} so they can be

gnu/services/mail.scm

@@ -1744,8 +1744,11 @@ match from local for any action outbound
            (documentation "Run the OpenSMTPD daemon.")
            (start (let ((smtpd (file-append package "/sbin/smtpd")))
                     #~(make-forkexec-constructor
-                       (list #$smtpd "-d" "-f" #$config-file)
-                       #:log-file #$log-file)))
+                       (list #$smtpd "-f" #$config-file
+                             #$@(if log-file '("-d") '()))
+                       #$@(if log-file
+                              (list #:log-file log-file)
+                              (list #:pid-file "/var/run/smtpd.pid")))))
            (stop #~(make-kill-destructor))))))
 
 (define %opensmtpd-accounts