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>

doc/fixes3-7-0.txt

@@ -1251,6 +1251,7 @@ use #monster to make dragon steed breathe
 wand of probing reveals tin contents
 steam vortices and fog clouds leave steam clouds behind
 fog clouds maintain any clouds they are in, even poisonous ones
+bone devils summon skeletons
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

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);