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/bgpd.h b/bgpd/bgpd.h
index 2b279cd..a6cc34e 100644
--- a/bgpd/bgpd.h
+++ b/bgpd/bgpd.h
@@ -747,6 +747,7 @@
 #define Receive_UPDATE_message                  12
 #define Receive_NOTIFICATION_message            13
 #define Clearing_Completed                      14
+#define BGP_Stop_with_error                     15
 #define BGP_EVENTS_MAX                          16
 
 /* BGP timers default value.  */