Merged multipath and single-path data structures.
Dropped struct mpnh and mpnh_*() Now struct nexthop exists, nexthop_*(), and also included struct nexthop into struct rta. Also converted RTD_DEVICE and RTD_ROUTER to RTD_UNICAST. If it is needed to distinguish between these two cases, RTD_DEVICE is equivalent to IPA_ZERO(a->nh.gw), RTD_ROUTER is then IPA_NONZERO(a->nh.gw). From now on, we also explicitely want C99 compatible compiler. We assume that this 20-year norm should be known almost everywhere.