Generate router_id automatically if possible (standard "smallest of local

regular interface addresses" rule).

Protocols should NOT rely on router_id existence -- when router ID is not
available, the router_id variable is set to zero and protocols requiring
valid router ID should just refuse to start, reporting such error to the log.

sysdep/unix/main.c

@@ -86,6 +86,7 @@ main(void)
   signal_init();
 
   scan_if_init();
+  auto_router_id();
 
   protos_start();