Fix translatability problems with composit text in aggregator feed generation

So I did a test translation run on the general strings extracted by potx-cli.php (which is still a bit buggy in this regard, so it did include aggregator stuff). Then I was puzzled by some of the source strings I needed to translate, and realized they are the result of misdirected usage of t() in composite text building. The problem is that aggregator module does things like:

$title = ' '. t('in category') .' '. check_plain($category->title);
$site_name = variable_get('site_name', 'Drupal');
... = $site_name .' '. t('aggregator')
..... = $site_name .' - '. t('aggregated feeds') . $title

This includes completely untranslatable stuff such as 'in category' and also has fixed word ordering and generic words like 'aggregator' used in a very specific context. So I modified these string building codes to use placeholders and full t() calls properly.

Please review.