Re-harden DBUS_SESSION_BUS_ADDRESS for AT_SECURE processes in GLib 2.70

Revert !1920 (merged) for the GLib 2.70 cycle. See #2305 (closed) and !1920 (merged) for details; in particular this comment:

Along the principle of “if not now, when?”, I’ll merge this now for 2.68 (and backport to 2.66), but will also file an issue for reverting it for 2.70 (i.e. hardening GLib fully again). That should give gnome-keyring/libsecret and msmtp time to rework their security. Note that at this point any attacker has most of the information they need to likely be able to construct a vulnerability in msmtp, so fixing it should ideally be a priority for them. I’ll file issues against gnome-keyring and msmtp with the details and timeline.