add usage examples

  +) 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.

     Usage example:

         EVP_MD_CTX md;

         EVP_MD_CTX_init(&md);             /* new function call */

         EVP_DigestInit(&md, EVP_sha1());

         EVP_DigestUpdate(&md, in, len);

         EVP_DigestFinal(&md, out, NULL);

         EVP_MD_CTX_cleanup(&md);          /* new function call */

     [Ben Laurie]

  +) Make DES key schedule conform to the usual scheme, as well as

     correcting its structure. This means that calls to DES functions

     now have to pass a pointer to a des_key_schedule instead of a

     plain des_key_schedule (which was actually always a pointer

     anyway).

     anyway): E.g.,

         des_key_schedule ks;

	 des_set_key_checked(..., &ks);

	 des_ncbc_encrypt(..., &ks, ...);

     (Note that a later change renames 'des_...' into 'DES_...'.)

     [Ben Laurie]

  *) Enhanced support for IA-64 Unix platforms (well, Linux and HP-UX).