caesar: Add SPDX-License-Identifier tags

[tuukka.pasanen_ilmi.fi]

Summary

Some BSD-3-Clause files under usr.bin/caesar were missing the
SPDX-License-Identifier-tag.

Sponsored by: The FreeBSD Foundation

[emaste]

I asked @tuukka.pasanen_ilmi.fi to put this one in review to facilitate discussion.

style(9) shows the SPDX tag coming after the copyright statement, not before as here. However, the example there does not have the additional text that this one does, so per style(9) should the SPDX tag go before the "This code is..." or before "Redistribution and ..."? Files that have two separate copyright blocks also make it somewhat unclear where it should go -- between the copyright statement and the license text in the first block?

It's very common in our tree to have the SPDX tag before the copyright statement and it seems there are many cases where that makes sense.

[emaste]

For cases where we aren't removing the license boilerplate text (because it has minor additions etc.) the SPDX tag should come first. SDPX comes after the copyright statement if we're omitting the boilerplate.

[imp]

So there's two fundamental cases that we have in the tree.

One is when there's no boilerplate. Here, we prefer, but don't require, copyright, spdx tag. Open source generally is split 70/30 with this order, hence our need to be flexible because different projects do different things, and even some parts of the same project are inconsistent.

The second case is when there is boilerplate. It's our practice in these cases to put the SPDX tag first in the file, as done in this review. We have an explicit policy that when there's both, the boilerplate text is controlling and the SPDX tag is informative. This case isn't as well documented as I'd like.

In fact, the discussion around this topic in other forums points to several stale docs in the tree that need to be updated to be consistent and refer to the canonical policy better, which I'm working on.