PPP: allow negotiating MRU with PPP_MRU config option

PPP_MRU is now free to be used for what it should have been. Now using
it at PPP init stage to set the wanted MRU value, triggering a MRU
negotiation at the LCP phase.

I doubt anyone needs it anyway, but, well, at least it is fixed and the
MRU/MTU config mess is cleaned.

And while we are at it, better document PPP MRU config values.

src/netif/ppp/lcp.c

@@ -373,7 +373,7 @@ static void lcp_init(ppp_pcb *pcb) {
 
     BZERO(wo, sizeof(*wo));
     wo->neg_mru = 1;
-    wo->mru = PPP_DEFMRU;
+    wo->mru = PPP_MRU;
     wo->neg_asyncmap = 1;
     wo->neg_magicnumber = 1;
     wo->neg_pcompression = 1;