PPP: introduce ppp_set_auth_required macro

PPP auth required flag is currently hardcoded to true if PPP is
acting as a server and set to false if PPP is acting as a client.

This is probably the most wanted behavior, but since we now have the
ability to change that at runtime, allow users to do it.

It means we can now have a server which asks the client to authenticate
or vice versa. This is pretty unusual thought. What we don't support
yet is mutual authentication with a different set of user and password
per direction which is even less usual.

doc/ppp.txt

@@ -323,6 +323,9 @@ ppp_set_ipcp_dnsaddr(ppp, 1, &addr);
 /* Auth configuration, this is pretty self-explanatory */
 ppp_set_auth(ppp, PPPAUTHTYPE_ANY, "login", "password");
 
+/* Require peer to authenticate */
+ppp_set_auth_required(ppp, 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).

src/include/netif/ppp/ppp.h

@@ -453,6 +453,13 @@ struct ppp_pcb_s {
 #define PPPAUTHTYPE_ANY       0xff
 void ppp_set_auth(ppp_pcb *pcb, u8_t authtype, const char *user, const char *passwd);
 
+#if PPP_AUTH_SUPPORT
+/*
+ * Whether peer is required to authenticate. This is mostly necessary for PPP server support.
+ */
+#define ppp_set_auth_required(ppp, boolval) (ppp->settings.auth_required = boolval)
+#endif /* PPP_AUTH_SUPPORT */
+
 #if PPP_IPV4_SUPPORT
 /*
  * Set PPP interface "our" and "his" IPv4 addresses. This is mostly necessary for PPP server

src/netif/ppp/pppos.c

@@ -351,12 +351,6 @@ pppos_listen(ppp_pcb *ppp, void *ctx)
   lcp_wo = &ppp->lcp_wantoptions;
   lcp_wo->silent = 1;
 
-#if PPP_AUTH_SUPPORT
-  if (ppp->settings.user && ppp->settings.passwd) {
-    ppp->settings.auth_required = 1;
-  }
-#endif /* PPP_AUTH_SUPPORT */
-
   /*
    * Default the in and out accm so that escape and flag characters
    * are always escaped.