Bug#1004448: gnome-session: should manage session with systemd --user, if available

[Simon McVittie]

Package: gnome-session
Version: 40.1.1-3
Severity: wishlist

Recent-ish upstream versions of gnome-session have been able to manage the
whole login session as a group of `systemd --user` services:
https://wiki.gnome.org/Initiatives/SystemdUser

In Debian terms, this requires libpam-systemd and dbus-user-session.

In particular, one benefit of this is that it makes gnome-shell able to
survive Xwayland crashing or being killed.

At the moment, we compile the code for this but do not enable it. It can
be enabled by hacking /usr/bin/gnome-session (or the session registration
.desktop file) to pass the --systemd option to the executable.

Alternatively, we could configure with it enabled by default.

To be nice to dbus-x11 and sysvinit users, we should make sure that
on systems lacking the prerequisites, this gracefully degrades to the
equivalent of `gnome-session --builtin` (the opposite of --systemd). I
think it already does, but haven't tried it yet.

    smcv