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/monst.h

@@ -201,4 +201,13 @@ struct monst {
 #define is_obj_mappear(mon,otyp) (M_AP_TYPE(mon) == M_AP_OBJECT \
                                   && (mon)->mappearance == (otyp))
 
+/* Get the maximum difficulty monsters that can currently be generated,
+   given the current level difficulty and the hero's level. */
+#define monmax_difficulty(levdif) (((levdif) + u.ulevel) / 2)
+#define monmin_difficulty(levdif) ((levdif) / 6)
+
+/* Macros for whether a type of monster is too strong for a specific level. */
+#define montoostrong(monindx, lev) (mons[monindx].difficulty > lev)
+#define montooweak(monindx, lev) (mons[monindx].difficulty < lev)
+
 #endif /* MONST_H */