Follow-up of the #44565 bug fix, renamed the misnamed
PPP_INPROC_MULTITHREADED to PPP_INPROC_IRQ_SAFE because it is
IRQ safe but not thread safe.
Updated PPP documentation which now clearly state when and how
this feature can be used.
Makes it clear we are initiating the PPP session with ppp_connect
(i.e. acting as a PPP client) so there is no confusion possible
between ppp_connect and ppp_listen.