bgpd: more SAFI fixes

bgpd: more SAFI fixes

Two macros resolving to the same integer constant broke a case block and
a more thorough merge of BGP_SAFI_VPNV4 and BGP_SAFI_VPNV6 was
performed.

* bgpd.h: MPLS-labeled VPN SAFI is AFI-independent, switch to single
* macro
* bgp_capability_test.c: update test data
* bgp_mp_attr_test.c: idem
* bgp_route.c: (bgp_maximum_prefix_overflow, bgp_table_stats_vty) update
  macro and check conditions (where appropriate)
* bgp_packet.c: (bgp_route_refresh_send, bgp_capability_send,
  bgp_update_receive, bgp_route_refresh_receive): idem
* bgp_open.c: (bgp_capability_vty_out, bgp_afi_safi_valid_indices,
  bgp_open_capability_orf, bgp_open_capability): idem
* bgp_attr.c: (bgp_mp_reach_parse, bgp_packet_attribute,
  bgp_packet_withdraw): idem