The interface for this is very similar to upstream, but the code is
quite different.

Support for “resuming” (i.e. calling |CMAC_Final| and then computing the
CMAC for an extension of the message) has been dropped. Also, calling
|CMAC_Init| with magic argument to reset it has been replaced with
|CMAC_Reset|.

Lastly, a one-shot function has been added because it can save an
allocation and that's what most callers actually appear to want to do.

Change-Id: I9345220218bdb16ebe6ca356928d7c6f055d83f6
Reviewed-on: https://boringssl-review.googlesource.com/4630
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>