exterror: advertise error strings via kern.features.exterr_strings

[netchild]

Summary

Allow userland, in particular test cases for EXTERROR conversions,
to detect at run time whether extended errors include the
descriptive message strings, which depends on the EXTERR_STRINGS
kernel option and cannot be probed in any other way.

MFC after: 1 week
Assisted-by: Claude Code (Fable 5)

Test Plan

unit test in another review

[netchild]

The kern.features.exterror part is for the benefit of userland programs which want to use it (e.g. difference between FreeBSD 14.x and 15.x).
The kern.features.exterr_strings part is for the benefit of unit tests to be able to skip if the verbose strings are not enabled.

[kib]

As I said it elsewhere, there is no point in kern.features.exterror. You cannot run 15 userspace on <= 14 kernel.

[netchild]

So you disagree with kern.features.exterror.
Do you also disagree with kern.features.exterr_strings which covers the optional behavior of verbose exterror and is useful in unit tests to differentiate between "fails because something is wrong -> error" and "fails because it does not exist -> intendet -> ok".

[kib]

In D58321#1337618, @netchild wrote:

So you disagree with kern.features.exterror.
Do you also disagree with kern.features.exterr_strings which covers the optional behavior of verbose exterror and is useful in unit tests to differentiate between "fails because something is wrong -> error" and "fails because it does not exist -> intendet -> ok".

kern.features.exterr_strings is fine

[kib]

Please upload patches with the full context.

share/man/man9/exterror.9

188

The example is rather pointless. I suggest to remove the lines 185-189.

[netchild]

full context