* common/ssh-utils.c (dummy_realloc): New function.
(dummy_free): Likewise.
(get_fingerprint): Prepend the fingerprint with the name of the digest
algorithm.  Correctly render SHA256-based ssh fingerprints.
* common/t-ssh-utils.c (sample_keys): Add SHA256 hashes for the keys.
(main): Add an option to dump the keys to gather fingerprints, also
print the SHA256 fingerprint for keys given as arguments, and check
the SHA256 fingerprints of the test keys.