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.
076f177100