ssh-keygen's available hashes are md5, sha1, sha256, sha384, and
sha512 (See digest-{openssl,libc}.c). ssh-keygen(1)'s man page
shows valid fingerprint hashes as only md5 and sha256. All these
hashes[1] were available when the man page declared only the subset
as valid. I'm able to use the others with the -E option but is
there a reason to not consider them valid?[1]: ripemd160 was available at that time but it's gone now.
