bgpd: make bgp_nlri_parse_encap conform with other nlri_parse funcs

* bgp_encap.{c,h} (bgp_nlri_parse_encap) afi is already in the NLRI argument.
  update or withdraw is signalled by attr being non-NULL or NULL.

* bgp_packet.c: (update_receive) fixup to match, and also make the attr
  argument conform with NLRI_ATTR_ARG for correct error handling on
  optional, transitive, partial, attributes.
3 files changed