Initial commit on integrated BIRD

New data types net_addr and variants (in lib/net.h) describing
network addresses (prefix/pxlen). Modifications of FIB structures
to handle these data types and changing everything to use these
data types instead of prefix/pxlen pairs where possible.

The commit is WiP, some protocols are not yet updated (BGP, Kernel),
and the code contains some temporary scaffolding.

Comments are welcome.

proto/ospf/rt.h

@@ -78,12 +78,13 @@ typedef struct ort
    * route was not in the last update, in that case other old_* values are not
    * valid.
    */
-  struct fib_node fn;
   orta n;
   u32 old_metric1, old_metric2, old_tag, old_rid;
   rta *old_rta;
   u8 external_rte;
   u8 area_net;
+
+  struct fib_node fn;
 }
 ort;