nixos/upterm: additional hardening

Before:
$ ps aux | grep upterm
root     2575046  0.0  0.0 1085080 6968 ?        Ssl  07:03   0:00 /nix/store/ci97r1lqx4128w75k7dcsw82j5bl0n3g-upterm-0.8.2/bin/uptermd --ssh-addr [::]:2323 --private-key ssh_host_ed25519_key

After
$ ps aux | grep upterm
uptermd  2832993  0.4  0.0 1158812 6856 ?        Ssl  07:08   0:00 /nix/store/ci97r1lqx4128w75k7dcsw82j5bl0n3g-upterm-0.8.2/bin/uptermd --ssh-addr [::]:2323 --private-key ssh_host_ed25519_key

nixos/modules/services/networking/uptermd.nix

@@ -85,6 +85,7 @@ in
         AmbientCapabilities = mkIf (cfg.port < 1024) [ "CAP_NET_BIND_SERVICE" ];
         CapabilityBoundingSet = mkIf (cfg.port < 1024) [ "CAP_NET_BIND_SERVICE" ];
         PrivateUsers = cfg.port >= 1024;
+        DynamicUser = true;
         LockPersonality = true;
         MemoryDenyWriteExecute = true;
         PrivateDevices = true;
@@ -95,7 +96,9 @@ in
         ProtectKernelLogs = true;
         ProtectKernelModules = true;
         ProtectKernelTunables = true;
-        RestrictAddressFamilies = [ "AF_INET" "AF_INET6" ];
+        ProtectProc = "invisible";
+        # AF_UNIX is for ssh-keygen, which relies on nscd to resolve the uid to a user
+        RestrictAddressFamilies = [ "AF_INET" "AF_INET6" "AF_UNIX" ];
         RestrictNamespaces = true;
         RestrictRealtime = true;
         SystemCallArchitectures = "native";