bgpd: consolidate attribute flag checks

bgpd: consolidate attribute flag checks

* bgpd/bgp_attr.c: (attr_flags_values []) array of required flags for
  attributes, EXTLEN & PARTIAL masked off as "dont care" as appropriate.
  (bgp_attr_flag_invalid) check if flags may be invalid, according to
  the above table & RFC rules.
  (bgp_attr_*) Use bgp_attr_flag_invalid.
  (bgp_attr_as4_aggregator) ditto, also take startp argument for the
  NOTIFY data.
  (bgp_attr_parse) pass startp to bgp_attr_as4_aggregator