Bug#635751: run as non-root

[Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>]

I think it would be nice to modify the service file to run as non-root:

[Service]
...
User=vnstat
CapabilityBoundingSet=

Untested.

Also it would be required to change the ownership of files in
/var/lib/vnstat, but this should be done by the install script.