[Ach] OpenSSH: hmac-ripemd160

[respiranto]

Hi,

I just noted the (recommended) hmac-ripemd160 MAC having become excluded
from the list of possible MACs in the OpenSSH 7.6 release.

Upgrading to OpenSSH 7.6, having enabled hmac-ripemd160, does cause sshd
to fail.

Unfortunately I don't know of the right way to add this information to
the document. If a new configuration for 7.6 was to be created, I assume
more things should be thought about (such as the note about Curve25519
being supported since 6.6p1 requires). The simple alternative would be
to add another such note.

Thanks,
respiranto