bgpd: add flag checks for MP_(UN)REACH_NLRI

* bgp_attr.[ch]
  * bgp_mp_reach_parse(): add extra arguments and a uniform flag
    check block
  * bgp_mp_unreach_parse(): idem
  * bgp_attr_parse(): provide extra arguments
* bgp_mp_attr_test.c
  * parse_test(): justify respective calls
3 files changed