courier: Rename Procmail to MDA

This patch renames courier.Procmail to courier.MDA, to make it more
obvious that the functionality is not tied to that particular MDA.

It's just for readability, there are no functional changes.

chasquid.go

@@ -159,7 +159,7 @@ func main() {
 	}
 	go stsCache.PeriodicallyRefresh(context.Background())
 
-	localC := &courier.Procmail{
+	localC := &courier.MDA{
 		Binary:  conf.MailDeliveryAgentBin,
 		Args:    conf.MailDeliveryAgentArgs,
 		Timeout: 30 * time.Second,