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/exper.c

@@ -215,7 +215,6 @@ const char *drainer; /* cause of death, if drain should be fatal */
         pline("%s level %d.", Goodbye(), u.ulevel--);
         /* remove intrinsic abilities */
         adjabil(u.ulevel + 1, u.ulevel);
-        reset_rndmonst(NON_PM); /* new monster selection */
     } else {
         if (drainer) {
             g.killer.format = KILLED_BY;
@@ -315,7 +314,6 @@ boolean incr; /* true iff via incremental experience growth */
         if (u.ulevelmax < u.ulevel)
             u.ulevelmax = u.ulevel;
         adjabil(u.ulevel - 1, u.ulevel); /* give new intrinsics */
-        reset_rndmonst(NON_PM);          /* new monster selection */
     }
     g.context.botl = TRUE;
 }