nixos/prosody: leverage systemd sandbox features to harden service

We are leveraging the systemd sandboxing features to prevent the
service accessing locations it shouldn't do. Most notably, we are here
preventing the prosody service from accessing /home and providing it
with a private /dev and /tmp.

Please consult man systemd.exec for further informations.

nixos/modules/services/networking/prosody.nix

@@ -856,6 +856,19 @@ in
         PIDFile = "/run/prosody/prosody.pid";
         ExecStart = "${cfg.package}/bin/prosodyctl start";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
+
+        MemoryDenyWriteExecute = true;
+        PrivateDevices = true;
+        PrivateMounts = true;
+        PrivateTmp = true;
+        ProtectControlGroups = true;
+        ProtectHome = true;
+        ProtectHostname = true;
+        ProtectKernelModules = true;
+        ProtectKernelTunables = true;
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
       };
     };