DSA SSH keys say "dss" in them, not "dsa".

Rather confusingly, DSA keys [have][rfc] the `ssh-dss` key type, not `ssh-dsa` as one might reasonably expect. Dropbear [can handle][db] these keys with no problem, but there's a bug in the web GUI preventing one from setting such keys. This patch fixes the problem.

[rfc]: http://tools.ietf.org/html/rfc4253#section-6.6
[db]: https://github.com/mkj/dropbear/blob/master/ssh.h#L104