agent: implement sd-notify reload correctly (#25636)

First of all, we should not send the unix time, but the monotonic time.
Second of all, RELOADING= and MONOTONIC_USEC fields should be sent in
*single* message not two separate messages.

From the man page of [systemd.service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Type=)

> notification message via sd_notify(3) that contains the "RELOADING=1" field in
> combination with "MONOTONIC_USEC=" set to the current monotonic time (i.e.
> CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as decimal string.

[sd_notify](https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html)
now has code samples of the protocol to clarify.

Without these changes, if you'd set
Type=notify-reload on the agen'ts systemd unit, systemd
would kill the service due to the service not responding to reload
correctly.

command/agent/command.go

@@ -978,16 +978,6 @@ func (c *Command) handleRetryJoin(config *Config) error {
 	return nil
 }
 
-// These constants are for readiness signalling via the systemd notify protocol.
-// The functions we send these messages to are no-op on non-Linux systems. See
-// also https://www.man7.org/linux/man-pages/man3/sd_notify.3.html
-const (
-	sdReady     = "READY=1"
-	sdReloading = "RELOADING=1"
-	sdStopping  = "STOPPING=1"
-	sdMonotonic = "MONOTONIC_USEC=%d"
-)
-
 // handleSignals blocks until we get an exit-causing signal
 func (c *Command) handleSignals() int {
 	signalCh := make(chan os.Signal, 4)
@@ -1026,8 +1016,7 @@ WAIT:
 
 	// Check if this is a SIGHUP
 	if sig == syscall.SIGHUP {
-		sdNotify(sdSock, sdReloading)
-		sdNotify(sdSock, fmt.Sprintf(sdMonotonic, time.Now().UnixMicro()))
+		sdNotifyReloading(sdSock)
 		c.handleReload()
 		sdNotify(sdSock, sdReady)
 		goto WAIT