ospf6d: copy "mtu-ignore" option from ospfd

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