bgp: use monotonic clock for time of day

bgp: use monotonic clock for time of day

BGP uses time() to get system time of day; but that value
fluctuates with time adjustments from NTP. This can cause premature
flapping of peer sessions and other failures.

Use the system monotonic clock supported by Quagga thread library
to avoid issue.

See: http://bugzilla.vyatta.com/show_bug.cgi?id=4467

* bgpd/bgp_fsm.c
  * bgp_uptime_reset(): dismiss function
* bgpd/bgpd.c
  * bgp_clock(): new function
* bgpd/bgp_damp.c
  * bgp_reuse_timer(): employ bgp_clock() instead of time(NULL)
  * bgp_damp_withdraw(): idem
  * bgp_damp_update(): idem
  * bgp_damp_scan(): idem
  * bgp_damp_info_vty(): idem
  * bgp_damp_reuse_time_vty(): idem
* bgpd/bgp_fsm.c
  * bgp_routeadv_timer(): idem
  * bgp_stop(): idem
  * bgp_establish(): idem
* bgpd/bgp_packet.c
  * bgp_update_receive(): idem
* bgpd/bgp_route.c
  * bgp_update_rsclient(): idem
  * bgp_update_main(): idem
  * bgp_static_update_rsclient(): idem
  * bgp_static_update_main(): idem
  * bgp_static_update_vpnv4(): idem
  * bgp_aggregate_route(): idem
  * bgp_aggregate_add(): idem
  * bgp_redistribute_add(): idem
* bgpd/bgp_snmp.c
  * bgpPeerTable(): idem
  * bgpTrapEstablished(): idem
  * bgpTrapBackwardTransition(): idem
* bgpd/bgpd.c
  * peer_create(): idem
  * peer_uptime(): idem
  * bgp_master_init(): idem