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.