cgls: mangle user-provided unit names

cgls: mangle user-provided unit names

so the CLI interface is now similar to `systemctl`, i.e. if no unit name
suffix is provided, assume `.service`.

Fixes: #20492

Before:
```
$ systemd-cgls --unit user@1000
Failed to query unit control group path: Invalid argument
Failed to list cgroup tree: Invalid argument
```

After:
```
$ build/systemd-cgls --unit user@1000
Unit user@1000.service (/user.slice/user-1000.slice/user@1000.service):
├─session.slice (#4939)
│ ├─pipewire-pulse.service (#5203)
│ │ └─7711 /usr/bin/pipewire-pulse
...
```