PPP: introduce ppp_set_passive and ppp_set_silent

There is two passive modes for PPPoS, passive more, for which we will
try to connect and then listen silently, and silent mode, for which we
will listen silently from the beginning.

Introduce ppp_set_passive and ppp_set_silent so the mode can be chosen
before connecting/listening.

doc/ppp.txt

@@ -326,6 +326,17 @@ ppp_set_auth(ppp, PPPAUTHTYPE_ANY, "login", "password");
 /* Require peer to authenticate */
 ppp_set_auth_required(ppp, 1);
 
+/*
+ * Only for PPPoS, the PPP session should be up and waiting for input.
+ *
+ * Note: for PPPoS, ppp_connect() and ppp_listen() are actually the same thing.
+ * The listen call is meant for future support of PPPoE and PPPoL2TP server
+ * mode, where we will need to negotiate the incoming PPPoE session or L2TP
+ * session before initiating PPP itself. We need this call because there is
+ * two passive modes for PPPoS, ppp_set_passive and ppp_set_silent.
+ */
+ppp_set_silent(pppos, 1);
+
 /*
  * Initiate PPP listener (i.e. wait for an incoming connection), can only
  * be called if PPP session is in the dead state (i.e. disconnected).