Introduce sys/kassert.h

It contains assert-related definitions previously provided by
sys/systm.h. The new header is leaner than whole systm.h.
Include kassert.h from systm.h for compatibility.

The copyright assignment to Eivind Eklund was suggested by Kirk McKusick
and is based in the commit 5526d2d920eb17b1507499f35b275b486f7fe8d0.

Suggested by: jhb
Reviewed by: alc, imp, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34089