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.

cmd/commandfuncs.go

@@ -269,10 +269,6 @@ func cmdRun(fl Flags) (int, error) {
 		}
 	}
 
-	if err := NotifyReadiness(); err != nil {
-		caddy.Log().Error("unable to notify readiness to service manager", zap.Error(err))
-	}
-
 	select {}
 }
 
@@ -294,15 +290,6 @@ func cmdReload(fl Flags) (int, error) {
 	reloadCmdAddrFlag := fl.String("address")
 	reloadCmdForceFlag := fl.Bool("force")
 
-	if err := NotifyReloading(); err != nil {
-		caddy.Log().Error("unable to notify reloading to service manager", zap.Error(err))
-	}
-	defer func() {
-		if err := NotifyReadiness(); err != nil {
-			caddy.Log().Error("unable to notify readiness to service manager", zap.Error(err))
-		}
-	}()
-
 	// get the config in caddy's native format
 	config, configFile, err := loadConfig(reloadCmdConfigFlag, reloadCmdConfigAdapterFlag)
 	if err != nil {