Start ssh-agent as a user unit

This has some advantages:

* You get ssh-agent regardless of how you logged in. Previously it was
  only started for X11 sessions.

* All sessions of a user share the same agent. So if you added a key
  on tty1, it will also be available on tty2.

* Systemd will restart ssh-agent if it dies.

* $SSH_AUTH_SOCK now points to the /run/user/<uid> directory, which is
  more secure than /tmp.

For bonus points, we should patch ssh-agent to support socket-based
activation...

nixos/modules/config/gnu.nix

@@ -36,7 +36,7 @@ with lib;
     # GNU lsh.
     services.openssh.enable = false;
     services.lshd.enable = true;
-    services.xserver.startOpenSSHAgent = false;
+    programs.ssh.startAgent = false;
     services.xserver.startGnuPGAgent = true;
 
     # TODO: GNU dico.