Disallow "." in session.name

PHP saves cookie with correct name, but upon deserialization to
$_COOKIE, it replaces "." characters with "_".

This is probably also reason why \SessionHandler is not able to find
a session.

https://harrybailey.com/2009/04/dots-arent-allowed-in-php-cookie-names/
https://bugs.php.net/bug.php?id=75883

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -339,7 +339,12 @@ private function addSessionSection(ArrayNodeDefinition $rootNode)
                     ->children()
                         ->scalarNode('storage_id')->defaultValue('session.storage.native')->end()
                         ->scalarNode('handler_id')->defaultValue('session.handler.native_file')->end()
-                        ->scalarNode('name')->end()
+                        ->scalarNode('name')
+                            ->validate()
+                                ->ifTrue(function ($v) { return false !== strpos($v, '.'); })
+                                ->thenInvalid('Session name can not contain character "."')
+                            ->end()
+                        ->end()
                         ->scalarNode('cookie_lifetime')->end()
                         ->scalarNode('cookie_path')->end()
                         ->scalarNode('cookie_domain')->end()