Encrypt-then-MAC support

[Yury Korolev]

Hi,

Any plans to support etm?

currently getting

`
kex error : no match for method mac algo client->server: server 
[[hmac-sha2-512-...@openssh.com](mailto:hmac-sha2-512-...@openssh.com),[hmac-sha2-256-...@openssh.com](mailto:hmac-sha2-256-...@openssh.com),[umac-128-...@openssh.com](mailto:umac-128-...@openssh.com)],
 client [hmac-sha2-256,hmac-sha2-512,hmac-sha1]
`

Thank you.