improve description of password prompting

Since the user does not know whether it will run su or sudo, indicate
whether the password prompt will be for root or the user's password,
when possible.

I assume that programs like gksu that can prompt for either depending on
system setup will make clear in their prompt what they're asking for.

Command/EnableTor.hs

@@ -49,10 +49,12 @@ start os = do
 			Just userid -> go uuid userid
 		else do
 			showStart "enable-tor" ""
-			showLongNote "Need root access to enable tor..."
 			gitannex <- liftIO readProgramFile
 			let ps = [Param (cmdname cmd), Param (show curruserid)]
-			ifM (liftIO $ runAsRoot gitannex ps)
+			sucommand <- liftIO $ mkSuCommand gitannex ps
+			maybe noop showLongNote
+				(describePasswordPrompt' sucommand)
+			ifM (liftIO $ runSuCommand sucommand)
 				( next $ next checkHiddenService
 				, giveup $ unwords $
 					[ "Failed to run as root:" , gitannex ] ++ toCommand ps