Support "ssh-dss" keys

Summary: Ref T4151. These are a (common) variant of "ssh-dsa" keys (which are somewhat theoretical, but show up on Google).

Test Plan: syntax

Reviewers: btrahan, dctrwatson, phpcodemonkey

Reviewed By: phpcodemonkey

CC: aran

Maniphest Tasks: T4151

Differential Revision: https://secure.phabricator.com/D7704