MINOR: systemd: Include MONOTONIC_USEC field in RELOADING=1 message

MINOR: systemd: Include MONOTONIC_USEC field in RELOADING=1 message

As per the `sd_notify` manual:

> A field carrying the monotonic timestamp (as per CLOCK_MONOTONIC) formatted
> in decimal in μs, when the notification message was generated by the client.
> This is typically used in combination with "RELOADING=1", to allow the
> service manager to properly synchronize reload cycles. See systemd.service(5)
> for details, specifically "Type=notify-reload".

Thus this change allows users with a recent systemd to switch to
`Type=notify-reload`, should they desire to do so. Correct behavior was
verified with a Fedora 39 VM.

see systemd/systemd#25916

[wla: the service file should be updated this way:]

    diff --git a/admin/systemd/haproxy.service.in b/admin/systemd/haproxy.service.in
    index 22a53d8aab..8c6dadb5e5 100644
    --- a/admin/systemd/haproxy.service.in
    +++ b/admin/systemd/haproxy.service.in
    @@ -8,12 +8,11 @@ EnvironmentFile=-/etc/default/haproxy
     EnvironmentFile=-/etc/sysconfig/haproxy
     Environment="CONFIG=/etc/haproxy/haproxy.cfg" "PIDFILE=/run/haproxy.pid" "EXTRAOPTS=-S /run/haproxy-master.sock"
     ExecStart=@SBINDIR@/haproxy -Ws -f $CONFIG -p $PIDFILE $EXTRAOPTS
    -ExecReload=@SBINDIR@/haproxy -Ws -f $CONFIG -c $EXTRAOPTS
    -ExecReload=/bin/kill -USR2 $MAINPID
     KillMode=mixed
     Restart=always
     SuccessExitStatus=143
    -Type=notify
    +Type=notify-reload
    +ReloadSignal=SIGUSR2

     # The following lines leverage SystemD's sandboxing options to provide
     # defense in depth protection at the expense of restricting some flexibility

Signed-off-by: William Lallemand <wlallemand@haproxy.com>