sshd now supports config file drop-ins, hence let's install one to hook
up "userdb ssh-authorized-keys", so that things just work.
We put the drop-in relatively early, so that other drop-ins generally
will override this.
Ideally sshd would support such drop-ins in /usr/ rather than /etc/, but
let's take what we can get. It's not that sshd's upstream was
particularly open to weird ideas from Linux people.
