Bug#1117610: gdm3: don't create a gdm user

[Jeremy Bícha]

Source: gdm3
Version: 49.0.1-1
X-Debbugs-CC: marco at ubuntu.com, alessandro.astone at canonical.com

This is an alternative to https://bugs.debian.org/1107944 . I figured
it is expressed differently enough to be worth a separate bug.

GDM 49 now uses dynamic users for greeter sessions. (By the way, the
merge request that landed this said that it allows gdm to be used with
dbus-broker instead of dbus-daemon).

Therefore, Arch Linux now installs gdm without a system user. It does
have a gdm **group** created with this sysusers snippet:
g gdm 120 -

That sets a UID for the group of 120. In Debian, I don't believe we
care about the gdm UID, so a Debian version would look like one of
these 2 lines, depending on whether we decide to keep the
Debian-specific group name:
g Debian-gdm - -
g gdm - -

Then there is this proposal to store dconf overrides in /etc/dconf/gdm.d/
https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/312

We could then remove our 91_dconf_database_path.patch. We have several
packaging files that reference dconf that would need to be updated.

Upgrades are an extra complication. In the long term, having a drop-in
directory for dconf overrides makes it simpler for users and system
administrators to override Debian defaults.

References
----------------
- https://gitlab.archlinux.org/archlinux/packaging/packages/gdm/-/blob/main/PKGBUILD
- https://src.fedoraproject.org/rpms/gdm/blob/rawhide/f/gdm.sysusers
Fedora 43 still creates a gdm user
- https://gitlab.gnome.org/GNOME/gdm/-/issues/553

Thank you,
Jeremy Bícha