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/do_wear.c

@@ -2213,19 +2213,26 @@ find_ac()
         uac -= u.ublessed;
     uac -= u.uspellprot;
 
-    /* [The magic binary numbers 127 and -128 should be replaced with the
-     * mystic decimal numbers 99 and -99 which require no explanation to
-     * the uninitiated and would cap the width of a status line value at
-     * one less character.]
-     */
-    if (uac < -128)
-        uac = -128; /* u.uac is an schar */
-    else if (uac > 127)
-        uac = 127; /* for completeness */
+    /* put a cap on armor class [3.7: was +127,-128, now reduced to +/- 99 */
+    if (abs(uac) > AC_MAX)
+        uac = sgn(uac) * AC_MAX;
 
     if (uac != u.uac) {
         u.uac = uac;
         g.context.botl = 1;
+#if 0
+        /* these could conceivably be achieved out of order (by being near
+           threshold and putting on +N dragon scale mail from bones, for
+           instance), but if that happens, that's the order it happened;
+           also, testing for these in the usual order would result in more
+           record_achievement() attempts and rejects for duplication */
+        if (u.uac <= -20)
+            record_achievement(ACH_AC_20);
+        else if (u.uac <= -10)
+            record_achievement(ACH_AC_10);
+        else if (u.uac <= 0)
+            record_achievement(ACH_AC_00);
+#endif
     }
 }