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.

caddy.go

@@ -32,6 +32,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/caddyserver/caddy/v2/notify"
 	"github.com/caddyserver/certmagic"
 	"github.com/google/uuid"
 	"go.uber.org/zap"
@@ -100,6 +101,16 @@ func Run(cfg *Config) error {
 // if it is different from the current config or
 // forceReload is true.
 func Load(cfgJSON []byte, forceReload bool) error {
+	if err := notify.NotifyReloading(); err != nil {
+		Log().Error("unable to notify reloading to service manager", zap.Error(err))
+	}
+
+	defer func() {
+		if err := notify.NotifyReadiness(); err != nil {
+			Log().Error("unable to notify readiness to service manager", zap.Error(err))
+		}
+	}()
+
 	return changeConfig(http.MethodPost, "/"+rawConfigKey, cfgJSON, forceReload)
 }