Revert "services: syslog: Simplify 'start' method."

This reverts commit 4c0cc7bed3.
Fixes <https://bugs.gnu.org/41116>.
Reported by Alex Sassmannshausen <alex@komputilo.eu>.

The problem is that reconfiguring on a system running Shepherd < 0.8.0
would lead to an error, due to #:file-creation-mask not being supported.
The error is rather harmless: it prevents service upgrade but the system
itself is upgraded.

We can reinstate that commit eventually, once #:file-creation-mask has
become widespread.

gnu/services/base.scm

@@ -1316,13 +1316,17 @@ (define syslog-service-type
       (documentation "Run the syslog daemon (syslogd).")
       (provision '(syslogd))
       (requirement '(user-processes))
-      (start #~(make-forkexec-constructor
+      (start #~(let ((spawn (make-forkexec-constructor
                              (list #$(syslog-configuration-syslogd config)
                                    "--rcfile"
                                    #$(syslog-configuration-config-file config))
+                             #:pid-file "/var/run/syslog.pid")))
+                 (lambda ()
                    ;; Set the umask such that file permissions are #o640.
-                #:file-creation-mask #o137
-                #:pid-file "/var/run/syslog.pid"))
+                   (let ((mask (umask #o137))
+                         (pid  (spawn)))
+                     (umask mask)
+                     pid))))
       (stop #~(make-kill-destructor))))))
 
 ;; Snippet adapted from the GNU inetutils manual.