luci-mod-network: add PREF64 support

Add support for ra_pref64 option for IPv6 Router Advertisements
options.
This option of IPv6 Router Advertisements allows devices connected to
a IPv6-only network to discover IPv6 prefix of the NAT64 gateway.
Devices can use this information for instance to setup client translator
(CLAT) from IPv4 to IPv6 in 464XLAT (RFC 6877) scenario or to handle
IPv4 address literal on application level.

Signed-off-by: Ondřej Caletka <ondrej@caletka.cz>
[ remove extra new line and fix commit description ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>