bug#46022: 27.1; kill-emacs should call sd_notify only in daemon mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Tim Ruffing <public@timruffing.de>
> Date: Thu, 21 Jan 2021 17:26:31 +0100
> 
> When run with --fg-daemon mode (as done by the provided systemd unit
> file "emacs.service"), emacs will call sd_notify() on startup to notify
> systemd about its status. On exit, emacs will call sd_notify()
> unconditionally, i.e., even when not run in daemon mode.
> 
> This means that when the daemon is running, other emacs processes
> derived from the main process (e.g., in batch mode started by the async
> package) may send exit notifications when they're exiting, even though
> the main daemon process is not exiting. systemd will (correctly) ignore
> these notifications because they come from the wrong PID; this is the
> lucky default value of "NotifyAccess=main" in the systemd unit, i.e.,
> only the main PID may send notifications. However, systemd will also
> emit a warning in this case. With my config, this results in a lot of
> warning messages in the systemd user journal:

I'm not an expert on systemd, not even close, but please review the
reasoning for what we do in bug#31498, and let's take it from there.
Quote:

  Both of these calls are successful no-ops if emacs was not started
  by systemd.

CC'ing Lucas, who submitted that changeset, in case hoe would like to
comment.