Use /etc/os-release for recent distros

[stormi]

/etc/os-release is a de-facto standard that describes a linux distribution, and has been around for years. See its announcement in 2012: http://0pointer.de/blog/projects/os-release and its manpage: https://www.freedesktop.org/software/systemd/man/os-release.html

See also some examples at https://gist.github.com/natefoo/814c5bf936922dad97ff

Using it, we could avoid parsing distro-specific release files, at least for distributions that are not extremely old.

It also comes with an interesting feature: the ID_LIKE key. Let's take Rocky Linux as an example, as we recently added support for it. If we had been using /etc/os-release:

1. Rocky Linux would automatically have been detected without any code addition.
2. In install.sh, we could have automatically inferred from ID_LIKE="rhel fedora" that we should install the xe-guest-utilities and xe-guest-utilities-xenstore RPMs.

[stormi]

This can be closed as #116 was merged.

My point 2. described above would be a different improvement, let's not keep the issue open for that.