devd: Warn for deprecated 'kern' system type

One year ago, I deprecated 'kern' in favor of 'kernel' for the system
name for some power events. I'm about to remove it from the kernel, but
realized there's been no warning generated for users. Preserve POLA by
converting on the fly here and issuing a warning for 14.x, and an fatal
error after we branch 15. Make compiling it an error on 16 to remove
the gross hack after we branch.

Sponsored by: Netflix
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D37584