propagate signals to the transferrer process group

Done on unix, could not implement it on windows quite.

The signal library gets part of the way needed for windows.
But I had to open https://github.com/pmlodawski/signal/issues/1 because
it lacks raiseSignal.

Also, I don't know what the equivilant of getProcessGroupIDOf is on
windows. And System.Process does not provide a way to send any signal to
a process group except for SIGINT.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Remote/BitTorrent.hs

@@ -181,7 +181,7 @@ tmpTorrentFile u = fromRepo . gitAnnexTmpObjectLocation =<< torrentUrlKey u
  - torrent file once.
  -}
 registerTorrentCleanup :: URLString -> Annex ()
-registerTorrentCleanup u = Annex.addCleanup (TorrentCleanup u) $
+registerTorrentCleanup u = Annex.addCleanupAction (TorrentCleanup u) $
 	liftIO . removeWhenExistsWith R.removeLink =<< tmpTorrentFile u
 
 {- Downloads the torrent file. (Not its contents.) -}