update an entry on EVP changes

     [Bodo Moeller, Lutz Jaenicke]

  *) Rationalise EVP so it can be extended: don't include a union of

     cipher/digest structures, add init/cleanup functions. This also reduces

     the number of header dependencies.

     cipher/digest structures, add init/cleanup functions for EVP_MD_CTX

     (similar to those existing for EVP_CIPHER_CTX).

     Usage example:

         EVP_MD_CTX md;