SSHd: add ETM MACs for SHA2

I am not a crypto-person at all, but why didn't you guys add the encrypt-then-mac versions of sha2 with 512 and 256, too? (*)

It should be still saver then the alternative `hmac-sha2-512` without etm.
**Note:** I am *not* sure if it should be *in front* or *after* `umac-128-etm` (I just guessed the first).

Furthermore, if you should aim for a maximum range of `Ciphers`, there are also `aes192-ctr` versions available as an intermediate option.

Last but not least: it might be worth mentioning on a side note, that one can also prefer Ciphers and MACs (and KexAlgorithms?) from the *client side* in `ssh_config` in case one has to connect to a machine with default config but strong crypto available.

(*) Should be in since 6.1 (but I tested it with OpenSSH 6.4).