notify: Send all sd_notify signals from main caddy process (#4060)

Initial sd_notify support was added in #3963, but that sent signals from
both cmdRun and cmdReload.  This approach has two drawbacks:

- Reloads initiated via the API do not send signals.
- The signals are sent from different processes, which requires the
  `NotifyAccess=exec` directive in the unit file.

This change moves the NotifyReloading and NotifyReadiness invocations to
Load, which address both of those drawbacks.  It also adds a
complimentary NotifyStopping method which is invoked from handleStop.
All the notify methods are defined in a notify package to avoid an
import loop.

admin.go

@@ -39,6 +39,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/caddyserver/caddy/v2/notify"
 	"github.com/caddyserver/certmagic"
 	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/zap"
@@ -905,6 +906,11 @@ func handleStop(w http.ResponseWriter, r *http.Request) error {
 			Err:        fmt.Errorf("method not allowed"),
 		}
 	}
+
+	if err := notify.NotifyStopping(); err != nil {
+		Log().Error("unable to notify stopping to service manager", zap.Error(err))
+	}
+
 	exitProcess(Log().Named("admin.api"))
 	return nil
 }