Refactor unit option declarations

This gets rid of some duplication.
Add support for user units

With ‘systemd.user.units’ and ‘systemd.user.services’, you can specify
units used by per-user systemd instances.  For example,

  systemd.user.services.foo =
    { description = "foo";
      wantedBy = [ "default.target" ];
      serviceConfig.ExecStart = "${pkgs.foo}/bin/foo";
    };

declares a unit ‘foo.service’ that gets started automatically when the
user systemd instance starts, and is stopped when the user systemd
instance stops.

Note that there is at most one systemd instance per user: it's created
when a user logs in and there is no systemd instance for that user
yet, and it's removed when the user fully logs out (i.e. has no
sessions anymore). So if you're simultaneously logged in via X11 and a
virtual console, you get only one copy of foo.
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...