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

[cem]

Summary

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

[markm]

I like this!

[cem]

In D19411#415710, @markm wrote:

I like this!

Thanks! :-)

[cem]

Need secteam/so/core blessing.

[cem]

Thanks!