Strictly speaking `tac` being unavailable doesn't imply `tail` has the `-r` option for "reverse"

[Korkman]

Effectively this is detecting absence of GNU coreutils and assuming BSD utils are present instead. It works, but isn't straight-forward.

Taking busybox into account (very slim distros like diet-pi deploy symlinks to busybox in place of coreutils), this should fail.

• Test some distro without coreutils, should fail
• Find a better solution, translate GNU / BSD / busybox utils into capabilities and polyfills