bgpd: implement admin distance

Until today the admin distance cannot be configured for any IPv6
routing protocol. This patch implements it for bgp.

Signed-off-by: Maitane Zotes <maz@open.ch>

patchwork #993: http://patchwork.quagga.net/patch/993/
diff --git a/bgpd/bgpd.h b/bgpd/bgpd.h
index 7665d9d..d8e687b 100644
--- a/bgpd/bgpd.h
+++ b/bgpd/bgpd.h
@@ -156,6 +156,11 @@
   u_char distance_ebgp;
   u_char distance_ibgp;
   u_char distance_local;
+
+  /* BGP ipv6 distance configuration.  */
+  u_char ipv6_distance_ebgp;
+  u_char ipv6_distance_ibgp;
+  u_char ipv6_distance_local;
   
   /* BGP default local-preference.  */
   u_int32_t default_local_pref;