`enable` option and fix for graceful shutdown

When a container is enabled a seperate service unit is created
to manage auto-starting of the container via the `wantedBy`
option. When disabled the container is still built but will not
be started by systemd, it can then be started/stopped manually
via `systemctl` or `nixos-container`.

Containers were not getting cleanly shutdown by systemd as
machinectl did not wait for container to shutdown and the
resulting RTMIN+4 signal would upset the service and trigger
systemd to start killing off the processes (I believe this needs
some work from upstream tbh). Now the container is shutdown via
the command socket, and machinectl is used to wait resulting in
a clean shutdown/exit.