RFC 4191 Default Router Preference support for router advertisements

Adds "ipv6 nd router-preference (high|medium|low)" and
"no ipv6 nd router-preference" interface commands.

Files modified:

   doc/ipv6.texi
   zebra/interface.c
   zebra/interface.h
   zebra/rtadv.c
   zebra/rtadv.h

Signed-off-by: Chris Caputo <ccaputo@alt.net>
diff --git a/zebra/rtadv.h b/zebra/rtadv.h
index abd1c6f..d8d263d 100644
--- a/zebra/rtadv.h
+++ b/zebra/rtadv.h
@@ -94,4 +94,6 @@
 } __attribute__((__packed__));
 #endif
 
+extern const char *rtadv_pref_strs[];
+
 #endif /* _ZEBRA_RTADV_H */