ospf6d: justify multicast group management

* ospf6_network.[ch]
  * ospf6_sso(): new function supersedes ospf6_join_allspfrouters(),
    ospf6_leave_allspfrouters(), ospf6_join_alldrouters() and
    ospf6_leave_alldrouters()
* ospf6_interface.c
  * ospf6_interface_state_change(): update respectively
  * interface_up(): ditto
  * interface_down(): ditto
diff --git a/ospf6d/ospf6_network.h b/ospf6d/ospf6_network.h
index fd8758e..0526b3e 100644
--- a/ospf6d/ospf6_network.h
+++ b/ospf6d/ospf6_network.h
@@ -35,11 +35,7 @@
 extern void ospf6_set_checksum (void);
 
 extern int ospf6_serv_sock (void);
-
-extern void ospf6_join_allspfrouters (u_int);
-extern void ospf6_leave_allspfrouters (u_int);
-extern void ospf6_join_alldrouters (u_int);
-extern void ospf6_leave_alldrouters (u_int);
+extern void ospf6_sso (u_int ifindex, struct in6_addr *group, int option);
 
 extern int ospf6_sendmsg (struct in6_addr *, struct in6_addr *,
                           unsigned int *, struct iovec *);