ppp: add reqprefix norelease ac_mac

ppp: add reqprefix norelease ac_mac

Use ppp and enable autoipv6, odhcp6c supports reqprefix and norelease to minimize the chance of IPv6 PD changes.[1]
PPPoE supports connecting to a specific AC/BRAS by using the pppoe-mac parameter.[2]

1. https://github.com/openwrt/openwrt/blob/master/package/network/ipv6/odhcp6c/files/dhcpv6.sh#L67
   https://github.com/openwrt/openwrt/blob/master/package/network/ipv6/odhcp6c/files/dhcpv6.sh#L77
2. https://github.com/ppp-project/ppp/blob/master/pppd/plugins/pppoe/plugin.c#L93

Signed-off-by: Andy Chiang <AndyChiang_git@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19978
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 7079e456ade5aad41e265c55cee49e4692807ecf)
Link: https://github.com/openwrt/openwrt/pull/20824
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>