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

include/extern.h

@@ -1216,7 +1216,6 @@ E void FDECL(dealloc_mextra, (struct monst *));
 E struct monst *FDECL(makemon, (struct permonst *, int, int, int));
 E boolean FDECL(create_critters, (int, struct permonst *, BOOLEAN_P));
 E struct permonst *NDECL(rndmonst);
-E void FDECL(reset_rndmonst, (int));
 E struct permonst *FDECL(mkclass, (CHAR_P, int));
 E struct permonst *FDECL(mkclass_aligned, (CHAR_P, int, ALIGNTYP_P));
 E int FDECL(mkclass_poly, (int));