[Icecast] Log to file + stderr

[Mikaël Fourrier]

Hi,

I would like to log the errors to stderr and to a file, but it doesn't 
seem possible for now. I tried

```
<errorlog>icecast-error.log</errorlog>
<errorlog>-</errorlog>
```

but the last setting erases the previous ones so I can only print to one 
of them. Could you add this feature?

Have a good day,
Mikaël Fourrier