Currently done for 40-autologin script only but might be
more widely useful: when describing an action to be done
while forming the LiveCD image, also prepare the one that
undoes the effect so that an installed LiveCD doesn't
(mis)behave as if it were young again.

NB: livecd-install provides 50-{gdm,kdm}-autologin-off.sh
hooks which can collide with ours, so let's override those
until things are sorted out properly at both sides.

PS: some half-year old nodm hacks are still in place for t6/branch
(and #27451 should be re-examined when dropping those).