cdefs.h: Introduce __nonstring attribute

This attribute can be used to annotate char arrays that are not supposed to
be terminated with a NUL char and is needed to silence clang's new
-Wunterminated-string-initialization warning. The name matches linuxkpi.

Reviewed by: emaste, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52565