bgpd: Add "no listen" socket option for the BGP master configuration

* bgpd.h: add a BGP_OPT_NO_LISTEN option for the master BGP configuration,
  to prevent any listen socket being created automatically. Allows code
  to be used outside of BGP daemon settings.
* bgpd.c: (bgp_get) honour above the flag, suppress auto-creation of listen
  socket on first BGP instance if set.
  (bgp_option_set) whitelist BGP_OPT_NO_LISTEN
diff --git a/bgpd/bgpd.h b/bgpd/bgpd.h
index bb30716..63e326a 100644
--- a/bgpd/bgpd.h
+++ b/bgpd/bgpd.h
@@ -59,6 +59,7 @@
 #define BGP_OPT_NO_FIB                   (1 << 0)
 #define BGP_OPT_MULTIPLE_INSTANCE        (1 << 1)
 #define BGP_OPT_CONFIG_CISCO             (1 << 2)
+#define BGP_OPT_NO_LISTEN                (1 << 3)
 };
 
 /* BGP instance structure.  */