Return with "-1" if parsing fails in read function.
Reduce buffersize of splitstring function.

Signed-off-by: Nick Hainke <vincent@systemli.org>

Getifaddrs() is not working all the time (e.g. wireguard interfaces).
Instead we use if_nameindex() syscall as suggested in:
 https://stackoverflow.com/a/45796495/8474618

Signed-off-by: Nick Hainke <vincent@systemli.org>

We scraped multiple times the same interface in one run.

Signed-off-by: Nick Hainke <vincent@systemli.org>

Add collectd plugin allowing to collect IPv6 interface statistics.

Signed-off-by: Nick Hainke <vincent@systemli.org>