PPP, CORE, ppp_close() and ppp_sighup() ended up sharing almost everything, merged

Merged ppp_sighup() to ppp_close() using an optional argument  "nocarrier"
on ppp_close().

doc/ppp.txt

@@ -256,7 +256,8 @@ ppp_set_default(ppp);
  * Initiate PPP negotiation, without waiting (holdoff=0), can only be called
  * if PPP session is in the dead state (i.e. disconnected).
  */
-ppp_open(ppp, 0);
+u16_t holdoff = 0;
+ppp_open(ppp, holdoff);
 
 
 /*
@@ -264,8 +265,13 @@ ppp_open(ppp, 0);
  * ======================
  */
 
-/* You can call this function anytime */
-ppp_close(ppp);
+/*
+ * Initiate the end of the PPP session, without carrier lost signal
+ * (nocarrier=0), meaning a clean shutdown of PPP protocols.
+ * You can call this function at anytime.
+ */
+u8_t nocarrier = 0;
+ppp_close(ppp, nocarrier);
 /*
  * Then you must wait your status_cb() to be called, it may takes from a few
  * seconds to several tens of seconds depending on the current PPP state.
@@ -354,3 +360,6 @@ from previous lwIP version is pretty easy:
 
 * If you used tcpip_callback_with_block() on ppp_ functions you may wish to use
   the PPPAPI API instead.
+
+* ppp_sighup and ppp_close functions were merged using an optional argument
+  "nocarrier" on ppp_close.