Update sshd MACs list

Rebased version of pull request #15.

Update `sshd`'s preferred MACs list in order to

a) favor stronger hash functions (`sha512` and `sha256`) over `sha1`
b) favor Encrypt-then-MAC (`*-etm`) modes over regular (Encrypt-and-MAC) modes.

The Encrypt-then-MAC modes were released as part of [OpenSSH 6.2](http://www.openssh.com/txt/release-6.2).