Default to creating system-wide connections if allowed

The IWD backend for NetworkManager does not support user-owned connections and will refuse to connect when trying to create such a connection. Since it is impossible to determine that the IWD backend is being used, instead just always default to creating system-wide connections if we are allowed to. This matches the behaviour of Gnome's network-manager-applet where the same change was made (for the same reason).

For reference, see https://gitlab.gnome.org/GNOME/network-manager-applet/-/merge_requests/98 , https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/743 and https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/845#note_1311483