cgi: Fix paths to static content

mfs.cgi wasn't working properly with Apache virtual directories. Example
of such a 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>

This commit fixes the CGI interface to access

  http://<host>/mfs/mfs.css

instead of:

  http://<host>/mfs.css

Change-Id: Ia9bfcba58c200385ce7592d342b9dd8d35b9ca29