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>

doxygen updates - include IPv6 in documentation. Exclude include/netif/ppp/polarssl.
Convert TODO -> @todo

PPP, move protocols initialization from ppp_clear to ppp_new

What protocols init functions are meant to is to be called once to set
the default configuration before user specific configuration is set.

Until now, we reset to the default configuration just before
reconnecting, thus without allowing any time frame to let users change
it. That was fine until one user asked to be able to do that.

This change move protocols init functions calls from ppp_clear to
ppp_new, meaning user configuration is not overwritten anymore.

PPP: Use LWIP_ARRAYSIZE at appropriate places

Use LWIP_ARRAYSIZE to simplify the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Sylvain Rochet <gradator@gradator.net>
[Sylvain Rochet <gradator@gradator.net>: added missing casts]
Signed-off-by: Sylvain Rochet <gradator@gradator.net>

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

PPP: more const and mixed u_char/char types fixes

PPP: fix constness in PPP related files when PRINTPKT_SUPPORT is enabled

PPP, sifnpmode() is only useful if on demand is supported, build out if not

We don't support PPP on demand, don't build sifnpmode() which is only
useful for on demand if on demand is not supported.

PPP, removed unnecessary memset()

Everything is cleared in ppp_clear(), we don't need to clear all
structures twice.

PPP, place print packet debug codenames in ROM

PPP, IPV6, correct fix for IPV6CP_COMP disabling

PPP, IPV6, fixed build if VJ support is disabled

PPP, IPV6CP, code cleaning on VJ support

PPP, IPV6CP, build out IPv6 VJ support using the existing IPV6CP_COMP define

PPP, IPv6, reduce size of llv6_ntoa() function

Reduced static buffer to the strict necessary (26 bytes), removed
call to eui64_ntoa(), merged eui64_ntoa() into llv6_ntoa().

272 bytes (code + static buffers) to 140 bytes on x86_64.

Improved eui64_ntoa() as well, we don't need it anymore but I'd
like to keep all eui64_* helpers functions.

PPP, IPv6, fixed unicode in file headers

PPP, CORE, Removed (*datainput) from struct protent if not used

Data input is only used by CCP and ECP, which are not supported at this time,
remove this entry from struct protent to save some flash.

PPP, CORE, Removed useless enabled_flag from struct protent

Our struct protent are const everywhere to save RAM, enable/disable
flag on a const struct is useless, saving some flash, removed.

PPP, from PPPD upstream, Eliminate some unnecessary ifdefs

PPP, from PPPD upstream, separate IPv6 handling for sifup/sifdown

The current code is buggy regarding handling of link state when using
both IPCP and IPv6CP: if IPv6CP has been set up and if during IPCP
negociation, ipcp_up() fails, it will incorrectly take the interface
down. The simple solution here is to change the platform code to do the
same as on Solaris: separate IPv6CP up/down state handling with sif6up()
and sif6down(), so that we really know when the interface is allowed to
go down.

(Based from pppd commit b04d2dc6df5c6b5650fea44250d58757ee3dac4a)

PPP, fixed more 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

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, checking for non-existent preprocessor macros is non-standard, added predefinition for all affected macros

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

PPP, adding const pragma to FSM callbacks, saving about 350 bytes in .data segment

PPP, adding const pragma to struct protent, saving about 200 bytes in .data segment

PPP, renamed all functions using common names in utils.c that can conflict with lwIP user code during link operation

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

some buggy compiler get confused with duplicated labels used in enum tcp_state and fsm.h, prepending PPP's ones

set LL addresses before bringing the interface up for IPv6

moved ipv6cp global variables to ppp_pcb

PPP IPv6 local link support added

added raw IPv6 PPP files