sshd: Do detach into the background

This is necessary to ensure that jobs that need to start after sshd
work properly.

This reverts 03f13a4939.

modules/services/networking/ssh/sshd.nix

@@ -347,10 +347,10 @@ in
 
         serviceConfig =
           { ExecStart =
-              "${pkgs.openssh}/sbin/sshd -D -h ${cfg.hostKeyPath} " +
+              "${pkgs.openssh}/sbin/sshd -h ${cfg.hostKeyPath} " +
               "-f ${pkgs.writeText "sshd_config" cfg.extraConfig}";
             Restart = "always";
-            Type = "simple";
+            Type = "forking";
             KillMode = "process";
             PIDFile = "/run/sshd.pid";
           };