Mostly this is a thin shim around existing code to integrate with
enc_xform and cryptosoft (+ cryptodev).
Note that for correctness, the enc_xform specifies an API blocksize of 1
byte. This is correct, but inefficient because our cryptosoft device
passes a single block at a time to algorithms. I intend to extend the
xform API a little bit here to enable saner chunking in future work.