Support an SSH error log

Summary: Ref T13624. Depends on D21578. In "sshd" subprocess contexts, use "PhutilErrorLog" to direct errors to both stderr and, if configured, a logfile on disk.

Test Plan:

• Confiugured an error log.
• Forced ssh-auth to fatal.
• Saw errors on stderr and in log.

Maniphest Tasks: T13624

Differential Revision: https://secure.phabricator.com/D21579