NET: Generic network statistics/estimator

Dave,

The following patchset introduces generic network statistics for netlink users. It uses nested TLV which prevents further compatibility problems when introducing new statistics. Backward compatibility to existing TLV types TCA_STATS and TCA_XSTATS is ensured but can be easly removed once it is no longer needed. Therefore prior users of struct tc_stats can be converted to this API and existing userspace applications will not notice a difference while converted applications can use the new extendable statistic interface.