AC and obj->spe limits: +127/-128 -> +99/-99

Cap overall AC at -99 instead of -128.  Put the same limit of 99
on enchantment and charge count of individual objects.

^X now reports if/when AC has reached its limit since players
could see that reaching that limit and then enchanting worn items
will change the worn items but not the total.  (Same thing would
have happened with -128, just without any explanation and less
likely to accomplish.)

Won't affect normal play for any reasonable definition of normal.

src/worn.c

@@ -472,6 +472,9 @@ register struct monst *mon;
             /* since ARM_BONUS is positive, subtracting it increases AC */
         }
     }
+    /* same cap as for hero [find_ac(do_wear.c)] */
+    if (abs(base) > AC_MAX)
+        base = sgn(base) * AC_MAX;
     return base;
 }