PPP, PPPoS, fixed thread safety of pppos_input()

PPPoS was actually not thread safe, pppos_input() can be called from
lwIP user port at any time, whatever the PPP state is. It might even be
called during pppos_connect() and pppos_listen(), this is quite unlikely
the port do that but nothing prevent the user to since we document
pppos_input() as being thread safe.

Added a mutex if PPP_INPROC_MULTITHREADED is set and ensure pppos_input()
is safe in regard to other pppos_* functions.

src/include/netif/ppp/pppos.h

@@ -67,6 +67,9 @@ typedef struct pppos_pcb_s pppos_pcb;
 struct pppos_pcb_s {
   /* -- below are data that will NOT be cleared between two sessions */
   ppp_pcb *ppp;                    /* PPP PCB */
+#if PPP_INPROC_MULTITHREADED
+  sys_mutex_t mutex;               /* Mutex for pppos_input() */
+#endif /* PPP_INPROC_MULTITHREADED */
   sio_fd_t fd;                     /* File device ID of port. */
 
   /* -- below are data that will be cleared between two sessions