Static: Support for multiple routes with the same network

Add support for proper handling of multiple routes with the same network
to the static protocol. Routes are distinguished by internal index, which
is assigned automatically (sequentially for routes within each network).
Having different route preference or igp_metric attribute is optional.

proto/static/static.h

@@ -40,6 +40,7 @@ struct static_route {
   struct static_route *mp_head;		/* First nexthop of this route */
   struct static_route *mp_next;		/* Nexthops for multipath routes */
   struct f_line *cmds;			/* List of commands for setting attributes */
+  uint index;				/* Distinguish different routes with same net */
   byte dest;				/* Destination type (RTD_*) */
   byte state;				/* State of route announcement (SRS_*) */
   byte active;				/* Next hop is active (nbr/iface/BFD available) */