Using libmnl instead of libnetlink for the netlink plugin

libnetlink has been removed from debian, and libmnl is suggested as an alternative. In any case, the plugin didn't build on any other linux architecture than i386, which is a shame.

Currently, the plugin builds, but segfaults except if built with `CFLAGS="-g -O0"`. More details here: https://gist.github.com/mfournier/6028636