exterror(9): split category into multiple sources

The static, global index into an array of strings is simple, cheap,
and works for the base kernel, but is unworkable for (potentially
third-party) kernel modules or for arbitrary userspace code. Swipe a
few of the top bits of category to indicate a source with all-zeros
being the current model (EXTERR_CAT_SRC_KERN_STATIC). Add two
additional sources EXTERR_CAT_SRC_KERN_DYNAMIC and EXTERR_CAT_SRC_USER
with stub implementations.

Reviewed by: kib
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D58236