`login` transition from src:shadow to src:util-linux

[Simon Chopin]

Hi folks,

While reviewing Skia's merge of src:shadow, we've noticed that the
`login` binary has been removed from it, and it is now provided by
util-linux (which Skia has also diligently merged).

Before I upload those merges, I wanted to touch base, especially since
this move is accompanied by a new Pre-Depends from login onto login.defs
(a new binary provided by src:shadow). Sadly, this change has not been
discussed on debian-devel at l.d.o.

>From all I know about Pre-Depends, preinst and conffile handling, and
with consulting with people with actual knowledge of this stuff (hi
Julian), this should all be fine, but in case you're seeing weirdness
happening that seems related to the topic, do get in touch. FWIW, Skia
and I have both been running them on our machines.

I'll be uploading the two packages Very Soon™.

Cheers,
Simon