This patch converts the seqiv IV generator to work with the new
AEAD interface where IV generators are just normal AEAD algorithms.

Full backwards compatibility is paramount at this point since
no users have yet switched over to the new interface.  Nor can
they switch to the new interface until IV generation is fully
supported by it.

So this means we are adding two versions of seqiv alongside the
existing one.  The first one is the one that will be used when
the underlying AEAD algorithm has switched over to the new AEAD
interface.  The second one handles the current case where the
underlying AEAD algorithm still uses the old interface.

Both versions export themselves through the new AEAD interface.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>