switch from System.Cmd.Utils to System.Process

Test suite now passes with -threaded!

I traced back all the hangs with -threaded to System.Cmd.Utils. It seems
it's just crappy/unsafe/outdated, and should not be used. System.Process
seems to be the cool new thing, so converted all the code to use it
instead.

In the process, --debug stopped printing commands it runs. I may try to
bring that back later.

Note that even SafeSystem was switched to use System.Process. Since that
was a modified version of code from System.Cmd.Utils, it needed to be
converted too. I also got rid of nearly all calls to forkProcess,
and all calls to executeFile, which I'm also doubtful about working
well with -threaded.

Remote/Bup.hs

@@ -136,9 +136,11 @@ retrieveEncrypted :: BupRepo -> (Cipher, Key) -> Key -> FilePath -> Annex Bool
 retrieveEncrypted buprepo (cipher, enck) _ f = do
 	let params = bupParams "join" buprepo [Param $ bupRef enck]
 	liftIO $ catchBoolIO $ do
-		(pid, h) <- hPipeFrom "bup" $ toCommand params
+		(_, Just h, _, pid)
+			<- createProcess (proc "bup" $ toCommand params)
+				{ std_out = CreatePipe }
 		withDecryptedContent cipher (L.hGetContents h) $ L.writeFile f
-		forceSuccess pid
+		forceSuccessProcess pid "bup" $ toCommand params
 		return True
 
 remove :: Key -> Annex Bool