vnstat: Update to 2.13

vnstat: Update to 2.13

For details see:
https://humdi.net/vnstat/CHANGES

"- Fixed
   - Opening of body html tag was missing on some pages in image output
     example cgi (examples/vnstat.cgi)
 - New
   - Add database data merge support as --merge
   - Add --db for specifying database file for queries (vnstat and vnstati)
   - Add exit status 2 options to --alert for making it possible to
     differentiate alerts from errors (exit status 1)
   - Add --dbiflist also to vnstati command
   - Image output example cgi (examples/vnstat.cgi) improvements
     - Remove dependency to vnstat command
     - Add option for selecting how many images are shown per row on the index
       page when the database has more than one interface
     - Add option for selecting which image output is used on the index page
       when the database has more than one interface
     - Add options for selecting which interfaces are shown or hidden from the
       index page without disabling access to all interface specific page when
       the database has more than one interface
     - Harmonize layout style between pages"

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>