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.c b/bgpd/bgpd.c
index bdc4e28..69c8c0a 100644
--- a/bgpd/bgpd.c
+++ b/bgpd/bgpd.c
@@ -82,6 +82,7 @@
     case BGP_OPT_NO_FIB:
     case BGP_OPT_MULTIPLE_INSTANCE:
     case BGP_OPT_CONFIG_CISCO:
+    case BGP_OPT_NO_LISTEN:
       SET_FLAG (bm->options, flag);
       break;
     default:
@@ -2064,7 +2065,8 @@
   *bgp_val = bgp;
 
   /* Create BGP server socket, if first instance.  */
-  if (list_isempty(bm->bgp))
+  if (list_isempty(bm->bgp)
+      && !bgp_option_check (BGP_OPT_NO_LISTEN))
     {
       if (bgp_socket (bm->port, bm->address) < 0)
 	return BGP_ERR_INVALID_VALUE;