examples: don't use deprecated OpenSSL hashing API

examples: don't use deprecated OpenSSL hashing API

The direct $HASH_{Init,Update,Final} has been discouraged for a while.
With the upcoming OpenSSL 3.0 it will be officially deprecated.

Add a handy macro, to avoid repetition and mistakes like in the current
code. Namely - we're using SHA cblock/digest_len for md5 :-\

The macro will also make it easier to add more, as seen with next patch.

v2: Align it with the crypto implementations, namely:
 - use openssh_hash_ctx::evp, use correct sizeof()
 - move hash_update out of the macro
 - remove forward declarations for hash functions

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>