Split out the sign/verify functionality from the existing akcipher
interface. Most algorithms in akcipher either support encryption
and decryption, or signing and verify. Only one supports both.
As a signature algorithm may not support encryption at all, these
two should be spearated.
For now sig is simply a wrapper around akcipher as all algorithms
remain unchanged. This is a first step and allows users to start
allocating sig instead of akcipher.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
