journalctl disk usage

**Problem:**
```sh
$ journalctl --disk-usage
Archived and active journals take up 7.2G in the file system.
```

**Solution:**
```sh
$ journalctl --vacuum-size=500M
$ journalctl --vacuum-time=5d

$ journalctl --disk-usage
Archived and active journals take up 192.0M in the file system.
```