improve thread termination handling

The reason the DirWatcher had to wait for program termination was because
it used withINotify, so when it finished, its watcher threads were killed.
But since I have two DirWatcher threads now, that was not good, and could
perhaps explain the MVar problem I saw yesterday. In any case, fixed this
part of the code by making the DirWatcher return a handle that can be used
to stop it, and now the main Assistant thread is the only one calling
waitForTermination.

Assistant.hs

@@ -73,6 +73,7 @@ import Assistant.Threads.Merger
 import Assistant.Threads.SanityChecker
 import qualified Utility.Daemon
 import Utility.LogFile
+import Utility.ThreadScheduler
 
 import Control.Concurrent
 
@@ -99,8 +100,8 @@ startDaemon assistant foreground
 				_ <- forkIO $ mergeThread st
 				_ <- forkIO $ daemonStatusThread st dstatus
 				_ <- forkIO $ sanityCheckerThread st dstatus changechan
-				-- Does not return.
-				watchThread st dstatus changechan
+				_ <- forkIO $ watchThread st dstatus changechan
+				waitForTermination
 
 stopDaemon :: Annex ()
 stopDaemon = liftIO . Utility.Daemon.stopDaemon =<< fromRepo gitAnnexPidFile