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

Move etharp to core/ipv4, which is a more appropriate place for it

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

PPP, PPPoX, added warnings about functions in headers which should not be called from lwIP user application

PPP, all protocols, uniformised naming

Uniformised fonction naming between and inside PPPoS, PPPoE, PPPoL2TP.

PPP, removed low level create functions from PPP core

ppp_over_ethernet_create() moved from ppp.c to pppoe.c
ppp_over_l2tp_create() moved from ppp.c to pppol2tp.c

PPP, moved ppp_write and ppp_netif_output low level protocols functions to respective low level protocol files

Moved ppp_write_over_ethernet() and ppp_netif_output_over_ethernet() to pppoe.c
Moved ppp_write_over_l2tp() and ppp_netif_output_over_l2tp() to pppol2tp.c

PPP: prepared callbacks from PPP core to low level protocols

New PPP callback from PPP core to low level protocols, first
step about removing named calls to low level protocols from PPP core.

PPP: removed PPPoE and PPPoL2TP callback status notifier

Added necessary PPP core functions for PPPoE and PPPoL2TP status
notificaton (ppp_link_failed and ppp_link_end), removed callback,
low level protocol are now calling PPP core "link" functions.

PPP: moved ppp_new() to low level protocol init for PPPoE and PPPoL2TP

First step of a rework of how low level protocols are using the
PPP core. Low level protocols are now going to use the core instead
of core using the low level protocols.

Final goal: separate PPP core code from low level protocols.

added optional macros PACK_STRUCT_FLD_8() and PACK_STRUCT_FLD_S() to prevent gcc 4 from warning about struct members that do not need packing

PPP, PPPoE, fixed bug #42138, pppoe_destroy() called with wrong pointer, PPPoE control block was never freed

PPP, don't allocate twice struct eth_hdr per PPPoE pbuf

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