feature: deprecate dsa

[shawn wilson]

Can we make the server stop creating ssh_host_dsa pairs (use them w/
another log line if they are manually created).

It would also be nice to add a big note at the top of the ssh-keygen
documentation about dsa being weak and that you shouldn't use it (if
you have a device that can only use dsa, you'll obviously ignore the
warning but it'd be good to say as strongly as possible).