The current code is buggy regarding handling of link state when using
both IPCP and IPv6CP: if IPv6CP has been set up and if during IPCP
negociation, ipcp_up() fails, it will incorrectly take the interface
down. The simple solution here is to change the platform code to do the
same as on Solaris: separate IPv6CP up/down state handling with sif6up()
and sif6down(), so that we really know when the interface is allowed to
go down.
(Based from pppd commit b04d2dc6df5c6b5650fea44250d58757ee3dac4a)
The lwIP PPP support is based from pppd 2.4.5 (http://ppp.samba.org) with
huge changes to match code size and memory requirements for embedded devices.
Anyway, pppd has a mature codebase for years and the average commit count
is getting low on their Git repositories, meaning that we can follow what
is happening on their side and merge what is relevant for lwIP.
So, here is the pppd follow up, so that we don't get away too far from pppd.
