Creates display text for teaser length option values.
Callback for drupal_map_assoc() within aggregator_form_aggregator_admin_form_alter().
$length: The desired length of teaser text, in bytes.
A translated string explaining the teaser string length.
function _aggregator_characters($length) {
return $length == 0 ? t('Unlimited') : format_plural($length, '1 character', '@count characters');
}