bgpd: Make aspath_highest 4-byte compatible for private AS

* bgp_aspath.h: Add BGP_AS_IS_PRIVATE macro.
* bgp_aspath.c: (aspath_highest) use said macro to also ensure 4-byte private
  AS range is ignored in calculating highest public ASN.
  (aspath_private_as_check) consolidate to use said macro.

Note: Extracted from 'bgpd: Add replace-as option to remove-private-as'
by paul@jakma.org.
2 files changed