bgpd: rewrite attr flag error logging

bgpd: rewrite attr flag error logging

* bgp_attr.c
  * attr_flag_str: new message list
  * bgp_attr_flags_diagnose(): new function, implements previously added
    error logging in a generic way
  * bgp_attr_origin(): use bgp_attr_flags_diagnose()
  * bgp_attr_nexthop(): ditto
  * bgp_attr_med(): ditto
  * bgp_attr_local_pref(): ditto
  * bgp_attr_atomic(): ditto
  * bgp_attr_originator_id(): ditto
  * bgp_attr_cluster_list(): ditto
  * bgp_mp_reach_parse(): ditto
  * bgp_mp_unreach_parse(): ditto