vnstat: Update to 1.11

Update vnstat to version 1.11, which also contains
the vnstati binary, for making graphs.

Remove the separate vnstati package.

There are bug fixes for the vnstati, which makes the graphs look nice when there are no data for an interface to a whole day.
There are also changes in the summary graphs.

This commit does not contain anything for doing
backups before upgrading, since I do not know
how that works.

The source for vnstat-1.11 has been downloaded from :
http://humdi.net/vnstat/vnstat-1.11.tar.gz
The changelog for vnstat-11 is here :
http://humdi.net/vnstat/CHANGES