PPP, DNS: introduce ppp_set_usepeerdns macro

PPP use peer DNS setting is currently hardcoded to true if PPP is
acting as a client and set to false if PPP is actinf as a server.

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

We don't have a way to have a different default configuration if the
PPP PCB is going to be used as a client or as a server, therefore the
default configuration should be fine for both of them. Since enabling
peer DNS by default is dangerous for server mode, the default is now
not to ask for DNS servers and it should now be explicitely enabled
if needed, update the documentation accordingly.

src/netif/ppp/ppp.c

@@ -627,10 +627,6 @@ ppp_pcb *ppp_new(struct netif *pppif, const struct link_callbacks *callbacks, vo
   memset(pcb, 0, sizeof(ppp_pcb));
 
   /* default configuration */
-#if LWIP_DNS
-  pcb->settings.usepeerdns = 1;
-#endif /* LWIP_DNS */
-
 #if PAP_SUPPORT
   pcb->settings.pap_timeout_time = UPAP_DEFTIMEOUT;
   pcb->settings.pap_max_transmits = UPAP_DEFTRANSMITS;

src/netif/ppp/pppos.c

@@ -357,11 +357,6 @@ pppos_listen(ppp_pcb *ppp, void *ctx)
   }
 #endif /* PPP_AUTH_SUPPORT */
 
-#if PPP_IPV4_SUPPORT && LWIP_DNS
-  /* Don't accept DNS from peer */
-  ppp->settings.usepeerdns = 0;
-#endif /* PPP_IPV4_SUPPORT && LWIP_DNS */
-
   /*
    * Default the in and out accm so that escape and flag characters
    * are always escaped.