Bone devils summon skeletons

Skeletons are extremely rare, and not generated at random,
and bone devils are basically just a speed bump when they appear.
Make both more interesting.

Idea by copperwater <aosdict@gmail.com>

src/minion.c

@@ -91,6 +91,9 @@ msummon(struct monst *mon)
                                                         : ndemon(atyp);
         cnt = ((dtype != NON_PM)
                && !rn2(4) && is_ndemon(&mons[dtype])) ? 2 : 1;
+    } else if (ptr == &mons[PM_BONE_DEVIL]) {
+        dtype = PM_SKELETON;
+        cnt = 1;
     } else if (is_ndemon(ptr)) {
         dtype = (!rn2(20)) ? dlord(atyp) : (!rn2(6)) ? ndemon(atyp)
                                                      : monsndx(ptr);