PPP: use mbed TLS sanitize functions if available

mbed TLS offers sanitize functions to zero-ise memory containing
cryptographic keys, use them if built with external mbed TLS library.

PPP: add a function map for hashes and ciphers to prepare for mbed TLS support

Unfortunately, all functions were renamed when PolarSSL was renamed to
mbed TLS, breaking the API. In order to continue supporting our embedded
PolarSSL copy while allowing our users to use mbed TLS, we need a function
map to deal with the API break.

This commit add a function map for all hashes and ciphers we are currently
using.

Work on task #13907 PPP cleanups: Move PPP options from opt.h to dedicated ppp_opts.h file

PPP: more const fixes

PPP: fix constness in PPP related files (GCC -Wcast-qual)

Signed-off-by: Dirk Ziegelmeier <dirk@ziegelmeier.net>

PPP, magic, renamed random_bytes() to magic_random_bytes() for API coherency

PPP, CHAP, reworked to pass ppp_pcb pointer to CHAP childs (MD5, MSCHAP, MSCHAPv2)

We are going to need ppp_pcb* in MSCHAP and MSCHAPv2 for MPPE for
int mppe_keys_set, u_char mppe_send_key and u_char mppe_recv_key
which are currently global variable which must be moved to ppp_pcb.

PPP, replaced drand48() with magic_pow()

PPP, added const modifier on auth strings

PPP, fixed some compiler warnings

Using -Wall -pedantic -Wparentheses -Wsequence-point
-Wswitch-default -Wextra -Wundef -Wshadow -Wpointer-arith
-Wbad-function-cast -Wc++-compat -Wwrite-strings -Wold-style-definition
-Wmissing-prototypes -Wredundant-decls -Wnested-externs -Wno-address

fixed bug #43235, cleared compiler warnings when using gcc with -Wextra option

PPP, switched chap_digests linked list to a const table in .rodata/flash, saving about 100 bytes in .data segment if chap(md5) and mschap is enabled

PPP, changed all the code enclosed between PPP_SERVER #if macro to our PPP PCB structure, making it easier to support PPP server in the future

Moved PPP headers into include/netif/ppp/, fixing bug #37040.

prevent conflict with existing (md4|md5|sha1|des) object file if polarssl is already used elsewhere in the project

EAP is now using ppp_pcb as well

PPP server support is now optional (disabled by default, not working until PPP have a "listen" support)

ppp_impl.h is back!

removed all system headers except the strict minimum

removed pppd.h and ppp.c, renamed pppmy.c to ppp.c and pppmy.h to ppp.h

added pppmy.h in all .c files to easily find conflicts

CHAP support is now an optional compile-time feature

Revert "Revert "Replaced md4/md5/sha1 implementations to PolarSSL ones""

This reverts commit ca2fd867b8e6b92ab4338c504e5b9567ab76e414.

Revert "Replaced md4/md5/sha1 implementations to PolarSSL ones"

This reverts commit aa2656cb9e8f6cdd7921fc36d5e00060065058a4.

Licence issue, we cannot include GPLed source code.

Replaced md4/md5/sha1 implementations to PolarSSL ones

Using cleaned PolarSSL md4/md5/sha1 implementations, without changing the API,
so that lwIP users already doing SSL or using PolarSSL don't need to compile
md4/md5/sha1 twice.

Added to that, we need a DES library for MSCHAP, and PolarSSL provided
a DES support.

And finally, PolarSSL is outstanding :-)

Early development stage of an attempt to port PPPd 2.4.5 to lwIP.

PPPoE works, PPPoS code is not ported at all.

I am using the RP-PPPoE server to do my tests using the following
configuration:

$ cat /etc/ppp/pppoe-server-options
debug
login
lcp-echo-interval 10
lcp-echo-failure 10
ms-dns 192.168.4.130
ms-dns 192.168.4.231
netmask 255.255.255.0
defaultroute
noipdefault
usepeerdns

$ cat /etc/ppp/allip
192.168.4.1-200

$ pppoe-server -C isp -L 192.168.4.254 -p /etc/ppp/allip -I tap0

Plus the usual auth-lines in /etc/ppp/pap-secrets and
/etc/ppp/chap-secrets .

And the unix port minimal "echo" project slightly modified to use
the "tcpip" API, so with threads, which I am going to commit with
NO_SYS as a -Dmacro.

It still use some of the linux'ism, such as syslog() and crypt(),

I do not want to drop the syslog() supports at the moment, this is
pretty useful to debug, and we may just convert the way the syslog() is
done to provide a trace feature to our PPP users, as a compile-time
option.