adacore git-hooks - daemon-mode email vs. systemd-logind linger

Hi, Joel -

As a part of more security review on sourceware, I had recently
experimented with systemd logind's KillUserProcesses=yes option.  It
turns out that this nuked one aspect adacore hooks' post-receive
processing, which create a background process to slowly dribble out
emails.

I restored KilledUserProcesses=no for now, but I'd really like to set
that back.  That means the hooks would have to change.  Here are a
couple of options:

- just send the emails immediately, without the daemon stuff; if the
  delays are there to try to sequentialize them, consider instead
  getting the hooks to emit Message-Id:/In-Reply-To:/Date: headers to
  let MUA's sort properly at reception
  
- invoke the email sending wrapped in a "systemd-run --user"
  deferred execution gadget, including a "sleep XXX" if you must
  keep time-hope-based sequencing

- move email stuff entirely out of the hooks; these repos are "public
  property" anyhow, and we can put cron jobs in place elsewhere to
  trigger email notifications about commits; heck, they could run the
  hook code itself later, just feed it retrospective git-hook lines

- FChE