Added PPPd follow-up file, so that we can track what is happening on pppd.

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.

src/netif/ppp/PPPD_FOLLOWUP

@@ -0,0 +1,170 @@
+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.
+
+
+== Patch fetched from from pppd Debian packages ==
+
+This has nothing to do with pppd, but we merged some good patch from
+Debian and this is a good place to be.
+
+- LCP adaptive echo, so that we don't send LCP echo request if we
+  are receiving data from peer, can be enabled by setting PPP_LCP_ADAPTIVE
+  to true.
+ 
+- IPCP no/replace default route option, were added in the early stage of
+  the ppp port, but it wasn't really helpful and was disabled when adding
+  the new API ppp_set_default() call, which gives the lwIP user control over
+  which one is the default interface, it was actually a requirement if you
+  are doing PPP over PPP (i.e. PPPoL2TP, VPN link, over PPPoE, ADSL link).
+
+- using rp-pppoe pppd exits with EXIT_OK after receiving a timeout waiting
+  for PADO due to no modem attached, bug reported to pppd bug tracker, fixed
+  in Debian but not in the latest (at the time where the port where started)
+  pppd release.
+
+
+== Commits on pppd ==
+
+2010-03-06 - Document +ipv6 and ipv6cp-accept-local
+  e7537958aee79b3f653c601e903cb31d78fb7dcc
+
+Don't care.
+
+
+2010-03-06 - Install pppol2tp plugins with sane permissions
+  406215672cfadc03017341fe03802d1c7294b903
+
+Don't care.
+
+
+2010-03-07 - pppd: Terminate correctly if lcp_lowerup delayed calling
+             fsm_lowerup
+  3eb9e810cfa515543655659b72dde30c54fea0a5
+
+Merged 2012-05-17.
+
+
+2010-03-07 - rp_pppoe: Copy acName and pppd_pppoe_service after option parsing
+  cab58617fd9d328029fffabc788020264b4fa91f
+
+Don't care, is a patch for pppd/plugins/rp-pppoe/plugin.c which is not part
+of the port.
+
+
+2010-08-23 - set and reset options to control environment variables
+             for scripts.
+  2b6310fd24dba8e0fca8999916a162f0a1842a84
+
+We can't fork processes in embedded, therefore all the pppd process run
+feature is disabled in the port, so we don't care about the new
+"environment variables" pppd feature.
+
+
+2010-08-23 - Nit: use _exit when exec fails and restrict values to 0-255
+             per POSIX.
+  2b4ea140432eeba5a007c0d4e6236bd0e0c12ba4
+
+Again, we are not running as a heavy process, so all exit() or _exit() calls
+were removed.
+
+
+2010-08-23 - Fix quote handling in configuration files to be more like shell
+             quoting.
+  3089132cdf5b58dbdfc2daf08ec5c08eb47f8aca
+
+We are not parsing config file, all the filesystem I/O stuff were disabled
+in our port.
+
+
+2010-08-24 - rp-pppoe: allow MTU to be increased up to 1500
+  fd1dcdf758418f040da3ed801ab001b5e46854e7
+
+Only concern changes on RP-PPPoE plugin, which we don't use.
+
+
+2010-09-11 - chat: Allow TIMEOUT value to come from environment variable
+  ae80bf833e48a6202f44a935a68083ae52ad3824
+
+See 2b6310fd24dba8e0fca8999916a162f0a1842a84.
+
+
+2011-03-05 - pppdump: Fix printfs with insufficient arguments
+  7b8db569642c83ba3283745034f2e2c95e459423
+
+pppdump is a ppp tool outside pppd source tree.
+
+
+2012-05-06 - pppd: Don't unconditionally disable VJ compression under Linux
+  d8a66adf98a0e525cf38031b42098d539da6eeb6
+
+Patch for sys-linux.c, which we don't use.
+
+
+2012-05-20 - Remove old version of Linux if_pppol2tp.h
+  c41092dd4c49267f232f6cba3d31c6c68bfdf68d
+
+Not in the port.
+
+
+2012-05-20 - pppd: Make MSCHAP-v2 cope better with packet loss
+  08ef47ca532294eb428238c831616748940e24a2
+
+This is an interesting patch. However it consumes much more memory for
+MSCHAP and I am not sure if the benefit worth it. The PPP client can
+always start the authentication again if it failed for whatever reason.
+
+
+2012-05-20 - scripts: Make poff ignore extra arguments to pppd
+  18f515f32c9f5723a9c2c912601e04335106534b
+
+Again, we are not running scripts.
+
+
+2012-05-20 - rp-pppoe plugin: Print leading zeros in MAC address
+  f5dda0cfc220c4b52e26144096d729e27b30f0f7
+
+Again, we are not using the RP-PPPoE plugin.
+
+
+2012-05-20 - pppd: Notify IPv6 up/down as we do for IPv4
+  845cda8fa18939cf56e60b073f63a7efa65336fc
+
+This is just a patch that adds plugins hooks for IPv6, the plugin interface
+was disabled because we don't have .so plugins in embedded.
+
+
+2012-05-20 - pppd: Enable IPV6 by default and fix some warnings
+  0b6118239615e98959f7e0b4e746bdd197533248
+
+Change on Makefile for IPv6, warnings were already cleared during port.
+
+
+2012-05-20 - contrib: Fix pppgetpass.gtk compilation
+  80a8e2ce257ca12cce723519a0f20ea1d663b14a
+
+Change on Makefile, don't care.
+
+
+2012-05-20 - pppd: Don't crash if crypt() returns NULL
+  04c4348108d847e034dd91066cc6843f60d71731
+
+We are using the PolarSSL DES implementation that does not return NULL.
+
+
+2012-05-20 - pppd: Eliminate some warnings
+  c44ae5e6a7338c96eb463881fe709b2dfaffe568
+
+Again, we are handling compilation warnings on our own.
+
+
+2012-05-20 - rp-pppoe plugin: Import some fixes from rp-pppoe-3.10
+  1817d83e51a411044e730ba89ebdb0480e1c8cd4
+
+Once more, we are not using the RP-PPPoE plugin.
+