vnstat2: fix all interfaces being monitored when none are configured

vnstat2: fix all interfaces being monitored when none are configured

By default, vnstatd adds all available interfaces on startup when its
database is empty. The --noadd option prevents this, but it breaks
import of legacy databases, and causes vnstatd to exit immediately
after startup, which breaks reloading.

This changes the init script to add the --noadd option when no legacy
databases need to be imported, and patches vnstatd to keep running
even when no interfaces are configured.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
(cherry picked from commit ecae7dedde0ccd2a636e93620ced41bca807d405)