This also stops using -1 as auth failed, and moves all failure statuses
to positive numbers. It looks like Exiting with -1 ends up setting the
status to 255 on exit, but then in the locker, it sees this as 255 and
not -1, since things get coerced into 32-bit integers.
By default it'll look at your environment to try to figure out which
display manager is used in order to start a new session. We first try
the org.freedesktop.DisplayManager dbus interface, and if that fails,
inspect XDG_SESSION_DESKTOP to try to figure out which display manager
is running.
The user can also still specify the correct display manager, or a custom
command.
This includes an abortive attempt to do a gtk4 dialog (which I don't
think is possible, as gtk4 doesn't allow embedding toplevels anymore),
and an iced dialog, which I just never started writing.