add 'extern "C" {}' for cplusplus in ppp headers

Cleanup hton*/ntoh* function handling and platform abstraction
Let lwip use functions/macros prefixed by lwip_ internally to avoid naming clashes with external #includes.
Remove over-complicated #define handling in def.h
Make functions easier to override in cc.h. The following is sufficient now (no more LWIP_PLATFORM_BYTESWAP):
#define lwip_htons(x) <your_htons>
#define lwip_htonl(x) <your_htonl>

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: fix constness in PPP related files when PRINTPKT_SUPPORT is enabled

PPP, removed useless padding in bitfields

Most of them were wrong actually. We have to use unsigned int because
C90 only allows int types for bitfields, so we are stuck to 32-bit
bitfields in most cases.

PPP, MPPE, drop unencrypted input packet if MPPE is required

PPP, MPPE, compile-out unused mppe_incomp() function

PPP, MPPE, reworked MPPE debug, using ppp_pcb metadata

PPP, MSCHAP, CCP, MPPE: reworked key passing from CCP to MPPE

Removed mutiple copies of keys by pre-setting MPPE keys during MSCHAP
negotiation.

Improved MPPE init so we don't need to pass a buffer formatted in a
special way to MPPE, this is necessary for pppd to talk to the kernel,
we don't need that here.

PPP, MSCHAP, MPPE: shared identical mppe_sha1_pad* between MSCHAP and MPPE

PPP, MPPE, struct ppp_mppe_state to typedef

We usually handle "typedefed" struct in lwIP, and I agree this is a cleaner
approach. Replaced struct ppp_mppe_state to ppp_mppe_state.

PPP, MPPE, optimized struct ppp_mppe_state size

PPP, MPPE, merged mppe_comp_init() and mppe_decomp_init() into mppe_init()

Removing a bit of redundancy and previous artefact of a generic kernel
interface. Exporting mppe_init() instead of mppe_comp_init() plus
mppe_decomp_init().

PPP, MPPE, removed mppe_alloc() and mppe_free()

We are going to use statically allocated struct ppp_mppe_state through PPP PCB,
removed now useless mppe_alloc() and mppe_free().

Merged mppe_alloc() key copy to mppe_init().

PPP, MPPE, changed API to use struct ppp_mppe_state* instead of void*

Now that struct ppp_mppe_state is exported, change MPPE API to use
struct ppp_mppe_state* instead of void*, we don't need that to be generic.

PPP, MPPE, moved struct ppp_mppe_state from mppe.c to mppe.h

In order to be able to use struct ppp_mppe_state statically allocated,
export struct ppp_mppe_state.

PPP, MPPE, adapt compressor to use pbuf

mppe_compress() now takes a pointer to a pbuf pointer and re-use the
passed buffer for MPPE "compression".

PPP, MPPE, removed address, control and protocol from compressor input

Our PPP stack deals with packet without address and control byte nor 2-byte
protocol field, improved mppe_compress() so we don't have to worry about
them.

PPP, MPPE, using err_t return for mppe_decompress

PPP, MPPE, adapt decompressor to use pbuf

mppe_decompress() now takes a pointer to a pbuf pointer and re-use the
passed buffer for MPPE "decompression". Removed sub protocol handling
which can be shared among all decompressors in ppp.c

PPP, MPPE, exported MPPE functions

PPP, MPPE, replaced #ifdef MPPE to #if MPPE_SUPPORT

PPP, CCP, various fix if MPPE is enabled