bgpd: improve NEXT_HOP attribute checks (BZ#680)

* lib/prefix.h
  * IPV4_CLASS_DE(): new helper macro
* bgp_attr.c
  * bgp_attr_nexthop(): add check for "partial" bit, refresh flag error
    reporting, explain meaning of RFC4271 section 6.3 and implement it

Conflicts:

	bgpd/bgp_attr.c
2 files changed