pkg/utils: Preserve the session class

In order to work with systemd-logind, a display manager is expected to
set the session class [1].  Setting the XDG_SESSION_CLASS environment
variable is one way to do that.

It's worth noting that the GNOME Display Manager (or GDM) does set
XDG_SESSION_CLASS [2] even though GNOME Shell uses other means to
differentiate between 'greeter' and 'user' sessions.

[1] https://www.freedesktop.org/wiki/Software/systemd/writing-display-managers/

[2] GDM commit 29ebbbb37bb65fad
    https://gitlab.gnome.org/GNOME/gdm/-/commit/29ebbbb37bb65fad
    https://bugzilla.gnome.org/show_bug.cgi?id=670100

#992

src/pkg/utils/utils.go

@@ -86,6 +86,7 @@ var (
 		"XDG_MENU_PREFIX",
 		"XDG_RUNTIME_DIR",
 		"XDG_SEAT",
+		"XDG_SESSION_CLASS",
 		"XDG_SESSION_DESKTOP",
 		"XDG_SESSION_ID",
 		"XDG_SESSION_TYPE",