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.
3 files changed