Pull request: all: add stub binary for new api

Merge in DNS/adguard-home from new-api to master

Squashed commit of the following:

commit 83f4418c253b9abc5131d9e2acc2a4a96e4122c4
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 26 19:09:34 2022 +0300

    all: fix build

commit 1fbb53fdf779bde79fab72f9c8eb929e08bb044c
Merge: 73a55197 1c89394a
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 26 18:37:27 2022 +0300

    Merge branch 'master' into new-api

commit 73a5519723f662979bdeb5192bc15835e7f03512
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 26 18:36:50 2022 +0300

    v1: imp names, docs

commit d3fbc2f2082612b8ba438c8216c6c74421cc2df5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Apr 22 17:55:42 2022 +0300

    cmd: imp docs

commit c2a73aa364a848e8066d1132d4b53bbc3e22db2d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Apr 22 16:19:14 2022 +0300

    all: add stub binary for new api

internal/v1/cmd/signal.go

@@ -0,0 +1,70 @@
+package cmd
+
+import (
+	"os"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghos"
+	"github.com/AdguardTeam/AdGuardHome/internal/v1/agh"
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// signalHandler processes incoming signals and shuts services down.
+type signalHandler struct {
+	signal chan os.Signal
+
+	// services are the services that are shut down before application
+	// exiting.
+	services []agh.Service
+}
+
+// handle processes OS signals.
+func (h *signalHandler) handle() {
+	defer log.OnPanic("signalProcessor.handle")
+
+	for sig := range h.signal {
+		log.Info("sigproc: received signal %q", sig)
+
+		if aghos.IsShutdownSignal(sig) {
+			h.shutdown()
+		}
+	}
+}
+
+// Exit status constants.
+const (
+	statusSuccess = 0
+	statusError   = 1
+)
+
+// shutdown gracefully shuts down all services.
+func (h *signalHandler) shutdown() {
+	ctx, cancel := ctxWithDefaultTimeout()
+	defer cancel()
+
+	status := statusSuccess
+
+	log.Info("sigproc: shutting down services")
+	for i, service := range h.services {
+		err := service.Shutdown(ctx)
+		if err != nil {
+			log.Error("sigproc: shutting down service at index %d: %s", i, err)
+			status = statusError
+		}
+	}
+
+	log.Info("sigproc: shutting down adguard home")
+
+	os.Exit(status)
+}
+
+// newSignalHandler returns a new signalHandler that shuts down svcs.
+func newSignalHandler(svcs ...agh.Service) (h *signalHandler) {
+	h = &signalHandler{
+		signal:   make(chan os.Signal, 1),
+		services: svcs,
+	}
+
+	aghos.NotifyShutdownSignal(h.signal)
+
+	return h
+}