fix "restore default" button, should be visible only when field has custom value

added missing validations for setup script
added two field descriptions
added SkipLockedTables to user preferences

libraries/user_preferences.inc.php

@@ -69,7 +69,7 @@ $msg->display();
 // warn about using session storage for settings
 $cfgRelation = PMA_getRelationsParam();
 if (!$cfgRelation['userconfigwork']) {
-    $msg = __('Your preferences will be saved only for current session. Storing them permanently requires %spmadb%s.');
+    $msg = __('Your preferences will be saved for current session only. Storing them permanently requires %spmadb%s.');
     $msg = PMA_sanitize(sprintf($msg, '[a@http://wiki.phpmyadmin.net/pma/pmadb@_blank]', '[/a]'));
     PMA_Message::notice($msg)->display();
 }