annex.jobs=cpus etc

Added the ability to run one job per CPU (core), by setting annex.jobs=cpus,
or using option --jobs=cpus or -Jcpus.

Built with future expansion in mind, including not defaulting matching on
Concurrency so more constructors can later be added, and using "cpu"
instead of "0".

Messages.hs

@@ -262,10 +262,12 @@ implicitMessage = whenM (implicitMessages <$> Annex.getState Annex.output)
  - the user.
  -}
 prompt :: Annex a -> Annex a
-prompt a = debugLocks $ go =<< Annex.getState Annex.concurrency
+prompt a = debugLocks $ Annex.getState Annex.concurrency >>= \case
+	NonConcurrent -> a
+	(Concurrent _) -> goconcurrent
+	ConcurrentPerCpu -> goconcurrent
   where
-	go NonConcurrent = a
-	go (Concurrent {}) = withMessageState $ \s -> do
+	goconcurrent = withMessageState $ \s -> do
 		let l = promptLock s
 		bracketIO
 			(takeMVar l)