Fixup: make signal handlers a bit simpler.

No need to spawn a goroutine, nor wait for a channel.
Let's just put everything in a single select call.

Signed-off-by: Xavier Nicollet <xnicollet@gmail.com>

cmd/alertmanager/main.go

@@ -548,14 +548,11 @@ func run() int {
 
 	var (
 		hup  = make(chan os.Signal, 1)
-		hupReady = make(chan bool)
 		term = make(chan os.Signal, 1)
 	)
 	signal.Notify(hup, syscall.SIGHUP)
 	signal.Notify(term, os.Interrupt, syscall.SIGTERM)
 
-	go func() {
-		<-hupReady
 	for {
 		select {
 		case <-hup:
@@ -563,15 +560,6 @@ func run() int {
 			_ = configCoordinator.Reload()
 		case errc := <-webReload:
 			errc <- configCoordinator.Reload()
-			}
-		}
-	}()
-
-	// Wait for reload or termination signals.
-	close(hupReady) // Unblock SIGHUP handler.
-
-	for {
-		select {
 		case <-term:
 			level.Info(logger).Log("msg", "Received SIGTERM, exiting gracefully...")
 			return 0