Giving both Torrents the same name has problematic side effects.

For example, if someone downloads both Torrents at the same time (USB image +
ISO):

 - The UI in a Bittorrent client does not allow to tell, at a glance, which
   is which.
 - If they use a BitTorrent client configured to download first to a temporary
   folder and then move the downloaded files elsewhere (e.g. kibi tells me most
   documentations for rtorrent recommend that), then when the 1st Torrent
   download completes, it breaks the 2nd one, deleting all files that were
   partially downloaded already.

So let's revert to having the Torrents download directories whose
names end respectively with -iso and -img.

This reverts most of commit a8964667, except the
part that got rid of permanent -iso and -img folders on the RM's filesystem,
which I assume is what made anonym unhappy with my initial implementation.

I've tried to salvage the hotfixes kibi and I had to do at release time, to fix
issues introduced by me while I did the first part of #16902 and later by anonym
in a89646.

I'll be the first one to go through the updated doc when I'll prepare 4.0~rc1
and 4.0, so I'll have the opportunity to notice and fix any breakage introduced
by this commit before it affects anyone else.