This will help us both for the GSettings migration and to implement the
fallback from GNOME Shell to Classic GNOME.

The --default-session-key argument has been changed to --session, which
takes the basename (without extension) of the .desktop-like file to use.
By default, gnome is used, and the gnome.session file is looked for in
$XDG_CONFIG_HOME/gnome-session/sessions,
$XDG_CONFIG_DIRS/gnome-session/sessions,
$XDG_DATA_DIRS/gnome-session/sessions.