fsjobqueue: pass accepted job types to New()

This makes the queue more type safe and allows to get rid of the
`pendingChannel` and `pendingChannels` helpers, which only existed to
create not-yet-existing pending channels.

cmd/osbuild-composer/main.go

@@ -125,7 +125,7 @@ func main() {
 		log.Fatalf("cannot create queue directory: %v", err)
 	}
 
-	jobs, err := fsjobqueue.New(queueDir)
+	jobs, err := fsjobqueue.New(queueDir, []string{"osbuild"})
 	if err != nil {
 		log.Fatalf("cannot create jobqueue: %v", err)
 	}