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.

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