make CommandStart return a StartMessage

The goal is to be able to run CommandStart in the main thread when -J is
used, rather than unncessarily passing it off to a worker thread, which
incurs overhead that is signficant when the CommandStart is going to
quickly decide to stop.

To do that, the message it displays needs to be displayed in the worker
thread, after the CommandStart has run.

Also, the change will mean that CommandStart will no longer necessarily
run with the same Annex state as CommandPerform. While its docs already
said it should avoid modifying Annex state, I audited all the
CommandStart code as part of the conversion. (Note that CommandSeek
already sometimes runs with a different Annex state, and that has not been
a source of any problems, so I am not too worried that this change will
lead to breakage going forward.)

The only modification of Annex state I found was it calling
allowMessages in some Commands that default to noMessages. Dealt with
that by adding a startCustomOutput and a startingUsualMessages.
This lets a command start with noMessages and then select the output it
wants for each CommandStart.

One bit of breakage: onlyActionOn has been removed from commands that used it.
The plan is that, since a StartMessage contains an ActionItem,
when a Key can be extracted from that, the parallel job runner can
run onlyActionOn' automatically. Then commands won't need to worry about
this detail. Future work.

Otherwise, this was a fairly straightforward process of making each
CommandStart compile again. Hopefully other behavior changes were mostly
avoided.

In a few cases, a command had a CommandStart that called a CommandPerform
that then called showStart multiple times. I have collapsed those
down to a single start action. The main command to perhaps suffer from it
is Command.Direct, which used to show a start for each file, and no
longer does.

Another minor behavior change is that some commands used showStart
before, but had an associated file and a Key available, so were changed
to ShowStart with an ActionItemAssociatedFile. That will not change the
normal output or behavior, but --json output will now include the key.
This should not break it for anyone using a real json parser.

Command/Multicast.hs

@@ -79,8 +79,7 @@ seek (MultiCastOptions Receive ups []) = commandAction $ receive ups
 seek (MultiCastOptions Receive _ _) = giveup "Cannot specify list of files with --receive; this receives whatever files the sender chooses to send."
 
 genAddress :: CommandStart
-genAddress = do
-	showStart' "gen-address" Nothing
+genAddress = starting "gen-address" (ActionItemOther Nothing) $ do
 	k <- uftpKey
 	(s, ok) <- case k of
 		KeyContainer s -> liftIO $ genkey (Param s)
@@ -91,7 +90,7 @@ genAddress = do
 	case (ok, parseFingerprint s) of
 		(False, _) -> giveup $ "uftp_keymgt failed: " ++ s
 		(_, Nothing) -> giveup $ "Failed to find fingerprint in uftp_keymgt output: " ++ s
-		(True, Just fp) -> next $ next $ do
+		(True, Just fp) -> next $ do
 			recordFingerprint fp =<< getUUID
 			return True
   where
@@ -123,7 +122,7 @@ parseFingerprint = Fingerprint <$$> lastMaybe . filter isfingerprint . words
 		in length os == 20
 	
 send :: [CommandParam] -> [FilePath] -> CommandStart
-send ups fs = withTmpFile "send" $ \t h -> do
+send ups fs = do
 	-- Need to be able to send files with the names of git-annex
 	-- keys, and uftp does not allow renaming the files that are sent.
 	-- In a direct mode repository, the annex objects do not have
@@ -131,47 +130,43 @@ send ups fs = withTmpFile "send" $ \t h -> do
 	-- expensive.
 	whenM isDirect $
 		giveup "Sorry, multicast send cannot be done from a direct mode repository."
-	
-	showStart' "generating file list" Nothing
-	fs' <- seekHelper LsFiles.inRepo =<< workTreeItems fs
-	matcher <- Limit.getMatcher
-	let addlist f o = whenM (matcher $ MatchingFile $ FileInfo f f) $
-		liftIO $ hPutStrLn h o
-	forM_ fs' $ \f -> do
-		mk <- lookupFile f
-		case mk of
-			Nothing -> noop
-			Just k -> withObjectLoc k (addlist f) (const noop)
-	liftIO $ hClose h
-	showEndOk
-
-	showStart' "sending files" Nothing
-	showOutput
-	serverkey <- uftpKey
-	u <- getUUID
-	withAuthList $ \authlist -> do
-		let ps =
-			-- Force client authentication.
-			[ Param "-c"
-			, Param "-Y", Param "aes256-cbc"
-			, Param "-h", Param "sha512"
-			-- Picked ecdh_ecdsa for perfect forward secrecy,
-			-- and because a EC key exchange algorithm is
-			-- needed since all keys are EC.
-			, Param "-e", Param "ecdh_ecdsa"
-			, Param "-k", uftpKeyParam serverkey
-			, Param "-U", Param (uftpUID u)
-			-- only allow clients on the authlist
-			, Param "-H", Param ("@"++authlist)
-			-- pass in list of files to send
-			, Param "-i", File t
-			] ++ ups
-		liftIO (boolSystem "uftp" ps) >>= showEndResult
-	stop
+	starting "sending files" (ActionItemOther Nothing) $
+		withTmpFile "send" $ \t h -> do
+			fs' <- seekHelper LsFiles.inRepo =<< workTreeItems fs
+			matcher <- Limit.getMatcher
+			let addlist f o = whenM (matcher $ MatchingFile $ FileInfo f f) $
+				liftIO $ hPutStrLn h o
+			forM_ fs' $ \f -> do
+				mk <- lookupFile f
+				case mk of
+					Nothing -> noop
+					Just k -> withObjectLoc k (addlist f) (const noop)
+			liftIO $ hClose h
+			
+			serverkey <- uftpKey
+			u <- getUUID
+			withAuthList $ \authlist -> do
+				let ps =
+					-- Force client authentication.
+					[ Param "-c"
+					, Param "-Y", Param "aes256-cbc"
+					, Param "-h", Param "sha512"
+					-- Picked ecdh_ecdsa for perfect forward secrecy,
+					-- and because a EC key exchange algorithm is
+					-- needed since all keys are EC.
+					, Param "-e", Param "ecdh_ecdsa"
+					, Param "-k", uftpKeyParam serverkey
+					, Param "-U", Param (uftpUID u)
+					-- only allow clients on the authlist
+					, Param "-H", Param ("@"++authlist)
+					-- pass in list of files to send
+					, Param "-i", File t
+					] ++ ups
+				liftIO (boolSystem "uftp" ps) >>= showEndResult
+			next $ return True
 
 receive :: [CommandParam] -> CommandStart
-receive ups = do
-	showStart' "receiving multicast files" Nothing
+receive ups = starting "receiving multicast files" (ActionItemOther Nothing) $ do
 	showNote "Will continue to run until stopped by ctrl-c"
 	
 	showOutput
@@ -204,7 +199,7 @@ receive ups = do
 				`after` boolSystemEnv "uftpd" ps (Just environ)
 		mapM_ storeReceived . lines =<< liftIO (hGetContents statush)
 		showEndResult =<< liftIO (wait runner)
-	stop
+	next $ return True
 
 storeReceived :: FilePath -> Annex ()
 storeReceived f = do