Add an optional "--sshd-key" argument to "bin/ssh-auth" for reading "%k" from modern sshd
- Used git pull locally, still worked fine.
- Instrumented things, saw the public key lookup actually work and emit a single key.
- Ran without "--sshd-key", got a full key list as before.
Maniphest Tasks: T13436
Differential Revision: https://secure.phabricator.com/D20874