Those are all consumed by our parser, so they all support comments.
I was considering whether they should have a license header at all,
but in the end I decided to add it because those files are often created
by copying parts of real unit files. And if the real ones have a license,
then those might as well. It's easier to add it than to make an exception.
Unfortunately we can't check the 6rd attribute, because it's not
exposed in /sys or anywhere other than netlink... But at least we
can check that networkd brings up an interface that looks right.