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/do.c

@@ -1453,7 +1453,6 @@ boolean at_stairs, falling, portal;
             || dunlev(&u.uz) < dunlev_reached(&u.uz))
             dunlev_reached(&u.uz) = dunlev(&u.uz);
     }
-    reset_rndmonst(NON_PM); /* u.uz change affects monster generation */
 
     /* set default level change destination areas */
     /* the special level code may override these */