convert to withCreateProcess for async exception safety

Not yet 100% done, so far I've grepped for waitForProcess and converted
everything that uses that to start the process with withCreateProcess.

Except for some things like P2P.IO and Assistant.TransferrerPool,
and Utility.CoProcess, that manage a pool of processes. See #2
in https://git-annex.branchable.com/todo/more_extensive_retries_to_mask_transient_failures/#comment-209f8a8c38e63fb3a704e1282cb269c7
for how those will need to be dealt with.

checkSuccessProcess, ignoreFailureProcess, and forceSuccessProcess calls waitForProcess, so
callers of them will also need to be dealt with, and have not been yet.

RemoteDaemon/Transport/Ssh.hs

@@ -37,14 +37,15 @@ transportUsingCmd cmd params rr@(RemoteRepo r gc) url h@(TransportHandle (LocalR
 
 transportUsingCmd' :: FilePath -> [CommandParam] -> Transport
 transportUsingCmd' cmd params (RemoteRepo r gc) url transporthandle ichan ochan =
-	robustConnection 1 $ do
-		(Just toh, Just fromh, Just errh, pid) <-
-			createProcess (proc cmd (toCommand params))
-			{ std_in = CreatePipe
-			, std_out = CreatePipe
-			, std_err = CreatePipe
-			}
-		
+	robustConnection 1 $ withCreateProcess p go
+  where
+	p = (proc cmd (toCommand params))
+		{ std_in = CreatePipe
+		, std_out = CreatePipe
+		, std_err = CreatePipe
+		}
+
+	go (Just toh) (Just fromh) (Just errh) pid = do
 		-- Run all threads until one finishes and get the status
 		-- of the first to finish. Cancel the rest.
 		status <- catchDefaultIO (Right ConnectionClosed) $
@@ -58,7 +59,8 @@ transportUsingCmd' cmd params (RemoteRepo r gc) url transporthandle ichan ochan
 		void $ waitForProcess pid
 
 		return $ either (either id id) id status
-  where
+	go _ _ _ _ = error "internal"
+
 	send msg = atomically $ writeTChan ochan msg
 
 	fetch = do