Fortuna: push CTR-mode loop down into randomdev hash.h interface

As a step towards adding other potential streaming ciphers. As well as just
pushing the loop down into the rijndael APIs (basically 128-bit wide AES-ICM
mode) to eliminate some excess explicit_bzero().

No functional change intended.

(cherry picked from commit 51c68d18e2580e5a3949b8c1b331125b71325e0b)