Added SETURIPRESENT and SETURIMISSING to external special remote protocol

Useful for things like ipfs that don't use regular urls.

An external special remote can add a regular url to a key, and then
git-annex get will download it from the web. But for ipfs, we want to
instead tell git-annex that the uri uses OtherDownloader. Before this
change, the external special remote protocol lacked a way to do that.

Logs/Web.hs

@@ -94,7 +94,7 @@ removeTempUrl key = Annex.changeState $ \s ->
 	s { Annex.tempurls = M.delete key (Annex.tempurls s) }
 
 data Downloader = WebDownloader | QuviDownloader | OtherDownloader
-	deriving (Eq)
+	deriving (Eq, Show)
 
 {- To keep track of how an url is downloaded, it's mangled slightly in
  - the log. For quvi, "quvi:" is prefixed. For urls that are handled by