cdefs: Add __deprecated

[imp]

Summary

Add __deprecated decorator. This is for a deprecated interface. We use
it today in our documentation, but don't actually provide it or use it.

Also, OpenZFS has a poorly-updated copy of compiler.h that we'll need
to fix upstream before this can land. Ideally, they'd be identical (and
OpenZFS would use the linuxkpi in FreeBSD version).

Sponsored by: Netflix

[jhb]

copystr (see copy(9)) might be a good candidate for this that might justify adding it?

[emaste]

The tense in the commit message confused me: when the commit lands we will provide __deprecated so it won't be correct to say we don't provide it.

[jhb]

In D46137#1051598, @emaste wrote:

The tense in the commit message confused me: when the commit lands we will provide __deprecated so it won't be correct to say we don't provide it.

Perhaps 's/don't actually/don't currently/' or 's/don't actually provide/haven't been providing or using it/'?

[imp]

Yea, I hadn't planned on including the second paragraph...

Also, just looked at Linux. It defines this as just a decorator. It used to provide warnings, but doesn't anymore.
So this could whine more than Linux does when we compile linuxkpi items that provide deprecated interfaces.

Also also: it can take an arg that's a message, so maybe we want to include that.

[imp]

Just posted to arch@