Add a 'native_blocksize' member to 'struct enc_xform' that stream ciphers
use to report their internal block size. cryptosoft will only pass in
native blocks to encrypt and decrypt hooks. For the final partial block,
'struct enc_xform' now has new encrypt_last/decrypt_last hooks which
accept the length of the final block. The multi_block methods are also
Mark AES-ICM (AES-CTR) as a stream cipher.