Bug#891599: gdm3: Greeter doesn't allow control of network-manager connections even when member of netdev group

[Matthew Gabeler-Lee]

Package: gdm3
Version: 3.22.3-3+deb9u1
Followup-For: Bug #891599

This actually looks like it might be a bug in gnome-shell, and still present
in 3.28.  This silly-looking(?) bit of logic is present there in
js/ui/status/network.js:

    _sessionUpdated() {
        let sensitive = !Main.sessionMode.isLocked && !Main.sessionMode.isGreeter;
        this.menu.setSensitive(sensitive);
    },

I.e. it looks like it's hard coding things to not allow interaction from the
greeter, no matter what permissions you might or might not have set.