exterror: advertise error strings via kern.features.exterr_strings

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.

Reviewed by: kib
MFC after: 1 week
Assisted-by: Claude Code (Fable 5)
Differential Revision: https://reviews.freebsd.org/D58321