passwd+LibCore: Make passwd replace /etc files atomically

Before this patch, we had a nasty race condition when changing a user's
password: there was a time window between truncating /etc/shadow and
writing out its new contents, where you could simply "su" to root
without using a password.

Instead of writing directly to /etc/passwd and /etc/shadow, we now
create temporary files in /etc and fill them with the new contents.
Those files are then atomically renamed to /etc/passwd and /etc/shadow.

Sadly, fixing this race requires giving the passwd program a lot more
privileges. This is something we can and should improve upon. :^)

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