Clean up CMAC implementation a little

Clean up CMAC implementation a little

Add explanatory comments that refer to the spec so that all the weird
dances make a little more sense. It turns out that this implmeentation
only supports block ciphers with block sizes of 64 and 128 bits, so
enforce this with a check.

Simplify make_kn() to make a little more sense and make it constant
time. Some stylistic fixes like checking pointers explicitly against
NULL and shuffle things into an order that makes a bit more sense.

Includes a fix for a warning reported by Viktor Szakats in
https://github.com/libressl/portable/issues/926

ok jsing