Peruse `/etc/os-release` among others when detecting system info

On Fedora, this results in a nice "Fedora 19" identifier and doesn't
show the codename "Schrödinger’s Cat" which is otherwise contained in
`/etc/fedora-release`.

On Arch, this shows "Arch Linux" where previously we had no info for it
(for some reason, `/etc/arch-release` is empty).

http://www.freedesktop.org/software/systemd/man/os-release.html