spamassassin: convert module to systemd

modules/services/mail/spamassassin.nix

@@ -52,7 +52,10 @@ in
 
     jobs.spamd = {
       description = "Spam Assassin Server";
-      startOn = "started networking and filesystem";
+
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" ];
+
       environment.TZ = config.time.timeZone;
       exec = "${pkgs.spamassassin}/bin/spamd ${optionalString cfg.debug "-D"} --username=spamd --groupname=spamd --nouser-config --virtual-config-dir=/var/lib/spamassassin/user-%u --allow-tell --pidfile=/var/run/spamd.pid";
     };