dtrace: Improve dmesg kernel message prefix

Provide intuitive log search keywords and increased system consistency.

Approved by: re (cperciva)
MFC after: 2 weeks
Reported by: mav
Reviewed by: 0mp, dteske
OpenZFS change: d45c8d648 (Improve dmesg kernel message prefix)
Differential Revision: https://reviews.freebsd.org/D55765

(cherry picked from commit 49e9b3061df036a73e8bc424c026158f3f57b183)
(cherry picked from commit b5f45474517646c3df377a75e50260026742ee92)