Add boolean option "bones"

Setting bones off prevents saving and loading bones files.

doc/Guidebook.mn

@@ -1927,6 +1927,8 @@ quality of the weapon; you are free to manually fill your quiver or quiver sack
 or make ready
 with the `Q' command instead.  If no weapon is found or the option is
 false, the `t' (throw) command is executed instead.  (default false)
+.lp bones
+Allow saving and loading bones files.  (default true)
 .lp boulder
 Set the character used to display boulders (default is rock class symbol).
 .lp catname