Make EVP_Digest*() routines return a value.

TODO: update docs, and make soe other routines
which use EVP_Digest*() check return codes.

Never use des_set_key (it depends on the global variable des_check_key),
but usually des_set_key_unchecked.
Only destest.c bothered to look at the return values of des_set_key,
but it did not set des_check_key -- if it had done so,
most checks would have failed because of wrong parity and
because of weak keys.

(meaning pointer to char) to des_cblock * (meaning pointer to
array with 8 char elements), which allows the compiler to
do more typechecking.  (The changed argument types were of type
des_cblock * back in SSLeay, and a lot of ugly casts were
used then to turn them into pointers to elements; but it can be
done without those casts.)

Introduce new type const_des_cblock -- before, the pointers rather
than the elements pointed to were declared const, and for
some reason gcc did not complain about this (but some other
compilers did).

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