Set a default session name for PHP sessions.

The default values are fine if no other applications use PHP sessions. However, when other apps are using PHP sessions, a conflict arises with the defaults, so let's try to make the defaults a bit more resilient.

config-templates/config.php

@@ -503,7 +503,7 @@ $config = array(
     /*
      * Options to override the default settings for php sessions.
      */
-    'session.phpsession.cookiename' => null,
+    'session.phpsession.cookiename' => 'SimpleSAML',
     'session.phpsession.savepath' => null,
     'session.phpsession.httponly' => true,