router: Add PREF64 (RFC 8781) support

router: Add PREF64 (RFC 8781) support

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.

To enable PREF64 option, a new uci parameter ra_pref64 has to contain
the NAT64 prefix, including prefix length. Only lengths of 96, 64, 56,
48, 40 and 32 bits are supported. For example, to annonce the Well-Known
Prefix:

config dhcp 'lan'
        …
        option ra_pref64 '64:ff9b::/96'

Fixes: #182
Signed-off-by: Ondřej Caletka <ondrej@caletka.cz>
[ remove extra space for Fixes tag ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>