Make EVP_Digest*() routines return a value.

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

sure they are available in opensslconf.h, by giving them names starting
with "OPENSSL_" to avoid conflicts with other packages and by making
sure e_os2.h will cover all platform-specific cases together with
opensslconf.h.

I've checked fairly well that nothing breaks with this (apart from
external software that will adapt if they have used something like
NO_KRB5), but I can't guarantee it completely, so a review of this
change would be a good thing.

Submitted by: Andy Polyakov <appro@fy.chalmers.se>