Add CBC-MAC authentication.

This adds the CBC-MAC code to the kernel, but does not hook it up to
anything (that comes in the next commit).

https://tools.ietf.org/html/rfc3610 describes the algorithm.

Note that this is a software-only implementation, which means it is
fairly slow.

Sponsored by: iXsystems Inc
Differential Revision: https://reviews.freebsd.org/D18592