logind: port over to use scopes+slices for all cgroup stuff

In order to prepare things for the single-writer cgroup scheme, let's
make logind use systemd's own primitives for cgroup management.

Every login user now gets his own private slice unit, in which his sessions
live in a scope unit each. Also, add user@$UID.service to the same
slice, and implicitly start it on first login.

src/systemctl/systemctl.c

@@ -3806,7 +3806,7 @@ static int snapshot(DBusConnection *bus, char **args) {
         if (!n)
                 return log_oom();
 
-        r = bus_method_call_with_reply (
+        r = bus_method_call_with_reply(
                         bus,
                         "org.freedesktop.systemd1",
                         "/org/freedesktop/systemd1",