`getFeedStats' calls the new static function `api_get_feed_stats' to
return an array of real feeds of the following form:
[{
"first" : 1127,
"unread" : 873,
"last" : 15460,
"title" : "Some feed",
"id" : 31,
"total" : 1513
}]
where "first", "last", "total" are the first, last and total number of
articles in the feed.
This adds the ability to nntp-oriented clients to efficiently retrieve
an "active file" without having to resort to `getHeadlines(limit=-1)'
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