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.

proto/bgp/attrs.c

@@ -1462,7 +1462,7 @@ static inline int
 rte_resolvable(rte *rt)
 {
   int rd = rt->attrs->dest;
-  return (rd == RTD_ROUTER) || (rd == RTD_DEVICE) || (rd == RTD_MULTIPATH);
+  return (rd == RTD_UNICAST);
 }
 
 int