cgi: Fix file paths

old pull request: #167

mfs.cgi not working properly with apache virtual directory

example Apache config:

```
Alias /mfs/ "/usr/share/mfscgi/"
<Directory "/usr/share/mfscgi/">
        AllowOverride None
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Order allow,deny
        Allow from all
        AddHandler cgi-script .cgi
</Directory>
```

currently Web browser tries to access the files
http://192.168.1.10/mfs.css
http://192.168.1.10/logomini.png

instead of: 
http://192.168.1.10/mfs/mfs.css
http://192.168.1.10/mfs/logomini.png

patch fixes a bug and for me works correctly with mfscgiserv and apache.