SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
Now that we've moved to atomic replacement of these files when altering
them, we don't need to keep them open for the lifetime of Core::Account
so just simplify this and close them when they are not needed.
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. :^)
