bgpd: refine the setting up of GTSM

* bgpd.h: Add error code for setting GTSM on iBGP
* bgpd.c: (peer_ttl_security_hops_set) use previous error code and signal
  incompatibility of GTSM+iBGP to vty.
  Consider the session state when setting GTSM, and reset Open/Active peers
  to let them pick up new TTL from start.
diff --git a/bgpd/bgpd.h b/bgpd/bgpd.h
index 39cdf8e..4da19e7 100644
--- a/bgpd/bgpd.h
+++ b/bgpd/bgpd.h
@@ -802,7 +802,8 @@
 #define BGP_ERR_CANNOT_HAVE_LOCAL_AS_SAME_AS    -28
 #define BGP_ERR_TCPSIG_FAILED			-29
 #define BGP_ERR_NO_EBGP_MULTIHOP_WITH_TTLHACK	-30
-#define BGP_ERR_MAX				-31
+#define BGP_ERR_NO_IBGP_WITH_TTLHACK		-31
+#define BGP_ERR_MAX				-32
 
 extern struct bgp_master *bm;