wireguard: when dyn-dns refresh is enabled, reconnect after failures

Make the dynamic-dns refresh systemd service (controlled via the
preexisting option dynamicEndpointRefreshSecond) robust to e.g. dns
failures that happen on intermittent network connections.

Background:

When dns resolution fails with a 'permanent' error ("Name or service not
known" instead of "Temporary failure in name resolution"), wireguard
won't retry despite WG_ENDPOINT_RESOLUTION_RETRIES=infinity.

-> This change should improve reliability/connectivity.

somewhat related thread: https://github.com/NixOS/nixpkgs/issues/63869

nixos/modules/services/networking/wireguard.nix

@@ -224,6 +224,21 @@ let
         '';
       };
 
+      dynamicEndpointRefreshRestartSeconds = mkOption {
+        default = null;
+        example = 5;
+        type = with types; nullOr ints.unsigned;
+        description = lib.mdDoc ''
+          When the dynamic endpoint refresh that is configured via
+          dynamicEndpointRefreshSeconds exits (likely due to a failure),
+          restart that service after this many seconds.
+
+          If set to `null` the value of
+          {option}`networking.wireguard.dynamicEndpointRefreshSeconds`
+          will be used as the default.
+        '';
+      };
+
       persistentKeepalive = mkOption {
         default = null;
         type = with types; nullOr int;
@@ -320,7 +335,16 @@ let
                 # cannot be used with systemd timers (see `man systemd.timer`),
                 # which is why `simple` with a loop is the best choice here.
                 # It also makes starting and stopping easiest.
+                #
+                # Restart if the service exits (e.g. when wireguard gives up after "Name or service not known" dns failures):
+                Restart = "always";
+                RestartSec = if null != peer.dynamicEndpointRefreshRestartSeconds
+                             then peer.dynamicEndpointRefreshRestartSeconds
+                             else peer.dynamicEndpointRefreshSeconds;
               };
+        unitConfig = lib.optionalAttrs dynamicRefreshEnabled {
+          StartLimitIntervalSec = 0;
+        };
 
         script = let
           wg_setup = concatStringsSep " " (