DSA-3488 libssh - security update

Aris Adamantiadis discovered that libssh, a tiny C SSH library,
incorrectly generated a short ephemeral secret for the
diffie-hellman-group1 and diffie-hellman-group14 key exchange methods.
The resulting secret is 128 bits long, instead of the recommended sizes
of 1024 and 2048 bits respectively. This flaw could allow an
eavesdropper with enough resources to decrypt or intercept SSH sessions.