[arch-general] Improve the filesystem package

[Yamakaky]

> And what happens if my root filesystem is mounted read-only? So, please let's
> not allow tmpfiles to touch /etc...

That's an interesting question, I don't know the answer. I think it will 
fail because ignoring the error would be worse for most of the cases.

> And all that assumes that you can execute systemd-sysusers, no?

systemd is the only init officially supported and partial updates are 
not supported so I think we can assume it will be available with this 
new version of the package.


After thinking, maybe removing the dirs from the package is not a so 
good idea. It reminds me the /usr/bin merge, which doesn't seem to be 
possible with tmpfiles.d. However, systemd-sysusers seems to be a good 
idea, especially for package creation. And stateless boot is cool.