opencrypto - Adjust xforms for explicit extra IV

opencrypto - Adjust xforms for explicit extra IV

* Some transforms such as XTS and CTR have some private 'IV'. Since our
  cryptosoft is now concurrent, we need to keep that private IV per-op
  and not per-session as OpenBSD does. We do this by passing in the
  local IV to encrypt() and decrypt() functions, as well as reinit(),
  which was already getting it.

* We can now also use AES-XTS and AES-CTR in a concurrent environment
  without having to have multiple sessions or an explicit key.

* While here, also radically simplify the reinit() for XTS.

* With these changes we can now use aes-xts for dm_target_crypt
  successfully.