ssh module: ignore exit code when socket activated

sshd will at times fail when exiting. When socket activated, this will
leave a number of sshd@ service instances in the failed state, so we
simply ignore the error code if we are running socket activated.

Recommended by upstream:
http://systemd-devel.freedesktop.narkive.com/d0eapMCG/socket-activated-sshd-service-showing-up-as-a-failure-when-the-client-connection-fails

Fixes: #3279

nixos/modules/services/networking/ssh/sshd.nix

···
 
             serviceConfig =
               { ExecStart =
+                  (optionalString cfg.startWhenNeeded "-") +
                   "${cfgc.package}/bin/sshd " + (optionalString cfg.startWhenNeeded "-i ") +
                   "-f ${pkgs.writeText "sshd_config" cfg.extraConfig}";
                 KillMode = "process";