bgpd: consolidate start timer backoff via a 'BGP_Stop_with_error' FSM event

* Exponential increase/back-off on the peer start timer is replicated in
  a few places. Consolidate by adding a "BGP_Stop_with_error" event
  so places outside FSM can just raise that event.
* bgpd.h: Add BGP_Stop_with_error
* bgp_fsm.c: (bgp_event_str[]) Add text for BGP_Stop_with_error.
  FSM table: Handle BGP_Stop_with_error, identical to bgp_stop
  in nearly all cases.
* bgp_packet.c: (bgp_write,bgp_write_notify) can just raise new event now.
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index f42e544..51b006a 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -766,15 +766,9 @@
 	  break;
 	case BGP_MSG_NOTIFY:
 	  peer->notify_out++;
-	  /* Double start timer. */
-	  peer->v_start *= 2;
-
-	  /* Overflow check. */
-	  if (peer->v_start >= (60 * 2))
-	    peer->v_start = (60 * 2);
 
 	  /* Flush any existing events */
-	  BGP_EVENT_ADD (peer, BGP_Stop);
+	  BGP_EVENT_ADD (peer, BGP_Stop_with_error);
 	  goto done;
 
 	case BGP_MSG_KEEPALIVE:
@@ -846,14 +840,7 @@
   /* Type should be notify. */
   peer->notify_out++;
 
-  /* Double start timer. */
-  peer->v_start *= 2;
-
-  /* Overflow check. */
-  if (peer->v_start >= (60 * 2))
-    peer->v_start = (60 * 2);
-
-  BGP_EVENT_ADD (peer, BGP_Stop);
+  BGP_EVENT_ADD (peer, BGP_Stop_with_error);
 
   return 0;
 }