PPP, PPPAPI: remove useless pppapi_set_auth function

Setting PPP authentication most only be done when the PPP PCB is in the
dead phase (i.e. disconnected). This is safe to access the PPP PCB
members while the session is down, therefore providing a thread-safe
function of it is meaningless and it might even be misleading.

All our new ppp_set_* functions do not have their equivalent
pppapi_set_* functions and they are not going to have them. At least
we make ppp_set_auth consistent with all others ppp_set_*, so that it
doesn't look like special.

PPP, PPPoS, SERVER: remove useless struct ppp_addrs* argument from pppos_listen

Now that we have helpers to set those members externaly, pppos_listen
struct ppp_addrs* argument does not add any value. In addition it
was not a well chosen design choice because the user needed to keep a
copy of struct ppp_addrs when listening again for a new connection.

Fix bug #47507: PPP API does not support LWIP_MPU_COMPATIBLE
I'd be glad if someone would test it :-)
Sylvain, if you don't like this patch feel free to revert it

PPP, fix reserved identifier violation (removed leading underscores from header include guards)

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

Move PPP API files to netif/ppp subdir