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_interface.h b/ospf6d/ospf6_interface.h
index 878c29e..cf758c0 100644
--- a/ospf6d/ospf6_interface.h
+++ b/ospf6d/ospf6_interface.h
@@ -76,6 +76,9 @@
   /* OSPF6 Interface flag */
   char flag;
 
+  /* MTU mismatch check */
+  u_char mtu_ignore;
+
   /* Decision of DR Election */
   u_int32_t drouter;
   u_int32_t bdrouter;