include: Implement N2867.

This adds macros for checked addition, subtraction, and multiplication with semantics similar to the builtins gcc and clang have had for years.

Reviewed by: kib, emaste
Differential Revision: https://reviews.freebsd.org/D41734

(cherry picked from commit e6615b10347caf67f5bc12c9a8e30b8ddd9860ae)

include: Add tests for N2867.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D41735

(cherry picked from commit 4fbb9c43aa44d9145151bb5f77d302ba01fb7551)

less: We have <stdckdint.h> now.

Reviewed by: delphij
Differential Revision: https://reviews.freebsd.org/D41736

(cherry picked from commit cb8dd292c7ec53391dfa25847858dd7ef895f94e)

Approved by: re (gjb)