KRT: Implemented asynchronous route / interface state notifications

(via Netlink). Tweaked kernel synchronization rules a bit. Discovered
locking bug in kernel Netlink :-)

Future plans: Hunt all the bugs and solve all the FIXME's.

sysdep/linux/krt-scan.c

@@ -127,6 +127,7 @@ krt_parse_entry(byte *ent, struct krt_proto *p)
 
   e = rte_get_temp(&a);
   e->net = net;
+  e->u.krt_sync.src = KRT_SRC_UNKNOWN;
   krt_got_route(p, e);
 }