ospf6d: copy "mtu-ignore" option from ospfd

"mtu-ignore" is an option ospfd used to mimic from the vendor's
implementation, now ospf6d will also implement it.

* ospf6_interface.h: extend ospf6_interface structure by one flag
* ospf6_interface.c: (ipv6_ospf6_mtu_ignore, no_ipv6_ospf6_mtu_ignore):
  new declarations; (ospf6_interface_create): show initial value for
  consistency; (ospf6_interface_show): print flag status
* ospf6_message.c: (ospf6_dbdesc_recv): consider interface-specific flag
  when checking MTU
diff --git a/ospf6d/ospf6_message.c b/ospf6d/ospf6_message.c
index 790fc0a..4648c45 100644
--- a/ospf6d/ospf6_message.c
+++ b/ospf6d/ospf6_message.c
@@ -832,7 +832,7 @@
     ((caddr_t) oh + sizeof (struct ospf6_header));
 
   /* Interface MTU check */
-  if (ntohs (dbdesc->ifmtu) != oi->ifmtu)
+  if (!oi->mtu_ignore && ntohs (dbdesc->ifmtu) != oi->ifmtu)
     {
       if (IS_OSPF6_DEBUG_MESSAGE (oh->type, RECV))
         zlog_debug ("I/F MTU mismatch");