adopt github pull request #286 - rndmonst()

Eliminate the cache that was supporting rndmonst() and pick a random
monster in a single pass through mons[] via "weighted reservoir
sampling", a term I'm not familiar with.

It had a couple of bugs:  if the first monster examined happened to
be given a weighting of 0, rn2() would divide by 0.  I didn't try
to figure out how to trigger that.  But the second one was easy to
trigger:  if all eligible monsters were extinct or genocided, it
would issue a warning even though the situation isn't impossible.

Aside from fixing those, the rest is mostly as-is.  I included a bit
of formatting in decl.c, moved some declarations to not require C99,
and changed a couple of macros to not hide and duplicate a call to
level_difficulty().

Fixes #286

src/read.c

@@ -2107,7 +2107,6 @@ do_class_genocide()
                      * have G_GENOD or !G_GENO.
                      */
                     g.mvitals[i].mvflags |= (G_GENOD | G_NOCORPSE);
-                    reset_rndmonst(i);
                     kill_genocided_monsters();
                     update_inventory(); /* eggs & tins */
                     pline("Wiped out all %s.", nam);
@@ -2324,7 +2323,6 @@ int how;
         } else if (ptr == g.youmonst.data) {
             rehumanize();
         }
-        reset_rndmonst(mndx);
         kill_genocided_monsters();
         update_inventory(); /* in case identified eggs were affected */
     } else {