force run container detached if container CreateCommand missing the detach param

the podman generated systemd service file has `Type=forking` service,
so the command after `ExecStart=` should not run in front.
if someone created a container and has the detach(`-d`) param missing
like this
```
podman create --name ngxdemo -P nginxdemos/hello
```
and generate the file with `--new` param:
```
podman generate systemd --name --new ngxdemo
```
because `podman run xxx` has no `-d` param,
so the container is not run in background and nerver exit.
and systemd will fail to start the service:
```
sudo systemctl start container-ngxdemo.service
Job for container-ngxdemo.service failed because a timeout was exceeded.
See "systemctl status container-ngxdemo.service" and "journalctl -xe" for details.
```

Signed-off-by: 荒野無燈 <ttys3@outlook.com>

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