lib/config, gui: Set unix socket permissions for GUI listen address (fixes #5979) (#6310)

gui/default/syncthing/core/syncthingController.js

@@ -2491,4 +2491,11 @@ angular.module('syncthing.core')
             $scope.config.options.crashReportingEnabled = enabled;
             $scope.saveConfig();
         };
+
+        $scope.isUnixAddress = function (address) {
+            return address != null &&
+                (address.startsWith('/') ||
+                    address.startsWith('unix://') ||
+                    address.startsWith('unixs://'));
+        }
     });

gui/default/syncthing/settings/settingsModalView.html

@@ -172,6 +172,13 @@
               </div>
             </div>
             <div class="col-md-6">
+              <div ng-if="isUnixAddress(tmpGUI.address)" class="form-group" ng-class="{'has-error': settingsEditor.UnixSocketPermissions.$invalid && settingsEditor.UnixSocketPermissions.$dirty}">
+                <label translate>UNIX Permissions</label>
+                <input id="UnixSocketPermissions" name="UnixSocketPermissions" class="form-control" type="text" ng-model="tmpGUI.unixSocketPermissions" ng-pattern="/^0?[0-7]{0,3}$/" />
+                <p class="help-block" ng-show="settingsEditor.UnixSocketPermissions.$invalid" translate>
+                  Enter up to three octal digits.
+                </p>
+              </div>
             </div>
           </div>
         </div>