fixes #2557 use pwg_get_cookie_var

-Update cookie to use pwg_lang to avoid conflicts with other potential apps.
-Add cookie path to the pwg_lang cookie to help with conflicts also.
-Be less restrictive in load_cookie_language function
-We can't use pwg_set_cookie because it is set in js not PHP so we force it to be called pwg_lang

password.php

@@ -499,7 +499,8 @@ foreach (get_languages() as $language_code => $language_name)
 
 $template->assign(array(
   'language_options' => $language_options,
-  'current_language' => $user['language']
+  'current_language' => $user['language'],
+  'COOKIE_PATH' => cookie_path(),
 ));
 
 //Get link to doc