[arch-general] Improve the filesystem package

[Yamakaky]

> Then, a filesystem update wants to add a new system user, so it
> updates the file /usr/lib/sysusers.d/archlinux.conf. The .install
> script runs systemd-sysusers and... nothing happens, because this
> program only creates the file when there is not there in the first
> place.

In fact no, it add the new user/group (I just tested). Try running 
`systemd-sysusers`, add a user in /usr/lib/sysusers.d/base.conf then run 
`systemd-sysusers`.

> The problem is similar. Once I have modified an existing file in
> /etc, what will happen when a new version of the file comes from the
> package. Will it not be installed? Or will it overwrite my own file?
> Maybe you can convice systemd-tmpfiles to copy the file to
> /etc/watever.pacsave, I don't know. But then, where is the
> advantage?

Yes, I didn't thought about pacnew, dummy I am... I wrote about 
/usr/share/etc for the stateless boot stuff (cf link from my previous 
mail), which is pretty cool. Maybe for this time we could let them in /etc.