#233873: reworked jittering of characters in image CAPTCHA: more robust to oversized fonts and less character overlap problems

updated captcha-module.pot
updated some po files

#213928: the list of unsolved challenges per user is now FIFO instead of "flush all when full", removes "You can't request more than @num challenges without solving them" message

backport of http://drupal.org/cvs?commit=90029 (added check to prevent "notice: Trying to get property of non-object ..." error message)
removed unused variable

solved regression bug introduced by http://drupal.org/cvs?commit=83921: font settings were moved to fieldset, but image_captcha_settings_form_pre_render() was not adapted to this