R1:888b3839e7e8

Prepare to route VCS connections through SSH

Summary:
Fixes T2229. This sets the stage for a patch similar to D7417, but for SSH. In particular, SSH 6.2 introduced an `AuthorizedKeysCommand` directive, which lets us do this in a mostly-reasonable way without needing users to patch sshd (if they have a recent enough version, at least).

The way the `AuthorizedKeysCommand` works is that it gets run and produces an `authorized_keys`-style file fragment. This isn't ideal, because we have to dump every key…