nixos/cloudflared: systemd dependency fix

- also wait for network-online.target:
fixes startup error on reboot without functioning networking
- restart on-failure (upstream service definition)

nixos/modules/services/networking/cloudflared.nix

···
             mkConfigFile = pkgs.writeText "cloudflared.yml" (builtins.toJSON fullConfig);
           in
           nameValuePair "cloudflared-tunnel-${name}" ({
-            after = [ "network.target" ];
+            after = [ "network.target" "network-online.target" ];
+            wants = [ "network.target" "network-online.target" ];
             wantedBy = [ "multi-user.target" ];
             serviceConfig = {
               User = cfg.user;
               Group = cfg.group;
               ExecStart = "${cfg.package}/bin/cloudflared tunnel --config=${mkConfigFile} --no-autoupdate run";
-              Restart = "always";
+              Restart = "on-failure";
             };
           })
         )