Containers: Fix reboot and poweroff

Previously "machinectl reboot/poweroff" brutally killed the container,
as did "systemctl stop/restart". And reboot didn't actually work. Now
everything is fine.
Containers: Don't remount /
Containers: Clean up veth interfaces
Containers: Use systemd-nspawn startup notification
Containers: Use systemd startup notification

Systemd in a container will call sd_notify when it has finished
booting, so we can use that to signal that the container is
ready. This does require some fiddling with $NOTIFY_SOCKET.