bgpd: improve attr flags checks

bgpd: improve attr flags checks

Do not check each of the Optional/Transitive/Partial attribute
flag bits, when their only valid combination is known in advance,
but still perform bit-deep error message logging. This change
assumes unused (low-order) 4 bits of the flag octet cleared.

* bgp_attr.c
  * bgp_attr_origin(): rewrite check
  * bgp_attr_nexthop(): idem
  * bgp_attr_med(): idem
  * bgp_attr_local_pref(): idem
  * bgp_attr_atomic(): idem