support VURL backend

Not yet implemented is recording hashes on download from web and
verifying hashes.

addurl --verifiable option added with -V short option because I
expect a lot of people will want to use this.

It seems likely that --verifiable will become the default eventually,
and possibly rather soon. While old git-annex versions don't support
VURL, that doesn't prevent using them with keys that use VURL. Of
course, they won't verify the content on transfer, and fsck will warn
that it doesn't know about VURL. So there's not much problem with
starting to use VURL even when interoperating with old versions.

Sponsored-by: Joshua Antonishen on Patreon

Remote/BitTorrent.hs

@@ -173,7 +173,7 @@ torrentUrlNum u
 
 {- A Key corresponding to the URL of a torrent file. -}
 torrentUrlKey :: URLString -> Annex Key
-torrentUrlKey u = return $ fromUrl (fst $ torrentUrlNum u) Nothing
+torrentUrlKey u = return $ fromUrl (fst $ torrentUrlNum u) Nothing False
 
 {- Temporary filename to use to store the torrent file. -}
 tmpTorrentFile :: URLString -> Annex RawFilePath