vnstat2: add hotplug script for adding interfaces

vnstat2: add hotplug script for adding interfaces

If an interface doesn't exist yet when vnStat is started, it won't be
monitored, as only existing interfaces can be added to the database via
the vnstat command.

This adds a hotplug script which adds any configured interfaces to the
vnStat database when it goes up.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
(cherry picked from commit f9ea8142601cee5542f9ba0f4e5a24e53ab59a6d)