Rework FreeBSD detection to use /etc/os-release + add testcases

[stormi]

Following #1, I'd like us to consider parsing or evaluating /etc/os-release rather than relying on the output of uname. This would also allow to add testcases to mk/testcases and then to contribute the detection of FreeBSD upstream.

This is about xe-linux-distribution only, not install.sh.

CCing @pacohope.

[pacohope]

That's a fine idea, and I'd be happy to do it. But if you look at the FreeBSD source it looks like os-release is a pretty new mechanism. I'm struggling to read the version history in SVN, but it looks like this was created around the time of 12.2-RELEASE. So if we drop uname support, I think we will lose support for some older versions (e.g., 11.X) that are presently supported. I like /etc/os-release as a primary, first choice mechanism, though.

[stormi]

I agree with you, we should test for/etc/os-release first, then fallback to uname if not present.