Make maddy buildable with GOOS=windows

Some people want to do so (#83) and it is not a big trouble
for us to support it.

maddy.go

@@ -2,9 +2,6 @@ package maddy
 
 import (
 	"io"
-	"os"
-	"os/signal"
-	"syscall"
 
 	"github.com/foxcpp/maddy/config"
 	"github.com/foxcpp/maddy/log"
@@ -100,27 +97,7 @@ func Start(cfg []config.Node) error {
 		}
 	}
 
-	sig := make(chan os.Signal, 5)
-	signal.Notify(sig, os.Interrupt, syscall.SIGTERM, syscall.SIGHUP, syscall.SIGINT, syscall.SIGUSR1)
-
-	for {
-		switch s := <-sig; s {
-		case syscall.SIGUSR1:
-			log.Println("SIGUSR1 received, reinitializing logging")
-			reinitLogging()
-		default:
-			log.Printf("signal received (%v), next signal will force immediate shutdown.", s)
-			// break inside switch exits switch, not outer loop
-			goto exitsigloop
-		}
-	}
-exitsigloop:
-
-	go func() {
-		s := <-sig
-		log.Printf("forced shutdown due to signal (%v)!", s)
-		os.Exit(1)
-	}()
+	waitForSignal()
 
 	for _, inst := range instances {
 		if closer, ok := inst.instance.(io.Closer); ok {