The arc4 algorithm has always had internal state.  It's been buggy
from day one in that the state has been stored in the shared tfm
object.  That means two users sharing the same tfm will end up
affecting each other's output, or worse, they may end up with the
same output.

Fix this by declaring an internal state and storing the state there
instead of within the tfm context.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>