machine_kill(): Don't kill the unit when killing the leader

If "machinectl poweroff" or "machinectl reboot" is used on a
systemd-nspawn container started with --keep-unit and --register, it
should *only* send the appropriate signal to the leader PID (i.e. the
container's systemd process). It shouldn't fall through to
manager_kill_unit() to also send the signal to the unit. The latter
ends up killing systemd-nspawn, which takes down the container
prematurely.