* ioctl.c: (if_get_mtu) set mtu6 to mtu
* mtu_kvm.c: (if_kvm_get_mtu) set mtu6 to mtu
* rt_netlink.c: (netlink_interface) set mtu6 to mtu
(netlink_link_change) ditto
* interface.c: (if_delete_update) only used with HAVE_NETLINK
and RTM_IFANNOUNCE.
(if_flag_dump_vty) Solaris IFF_IPV4 and IFF_IPV6 if flags
(if_dump_vty) print mtu6 if not same as mtu