bgpd: justify checks for IPv4 class D/E

* lib/prefix.h
  * IPV4_CLASS_DE(): make consistent with counterpart macros
* bgp_packet.c
  * bgp_open_receive(): test using macro instead of ">="
* bgp_route.c
  * bgp_update_rsclient(): idem
  * bgp_update_main(): idem
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index 5f15060..fc653e2 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -1387,7 +1387,7 @@
 
   /* remote router-id check. */
   if (remote_id.s_addr == 0
-      || ntohl (remote_id.s_addr) >= 0xe0000000
+      || IPV4_CLASS_DE (ntohl (remote_id.s_addr))
       || ntohl (peer->local_id.s_addr) == ntohl (remote_id.s_addr))
     {
       if (BGP_DEBUG (normal, NORMAL))