Tweak wand of make invisible and potion of invisibility

Wand of make invisible doesn't make you permanently invisible,
just for a short duration.  Potion of invisibility makes you
invisible for much longer period, or if blessed, has a small
chance of giving permanent invisibility.

This makes the wand actually useful, and improves the spell
too.

src/potion.c

@@ -806,10 +806,10 @@ peffect_invisibility(struct obj *otmp)
     } else {
         self_invis_message();
     }
-    if (otmp->blessed)
+    if (otmp->blessed && !rn2(HInvis ? 15 : 30))
         HInvis |= FROMOUTSIDE;
     else
-        incr_itimeout(&HInvis, rn1(15, 31));
+        incr_itimeout(&HInvis, d(6 - 3 * bcsign(otmp), 100) + 100);
     newsym(u.ux, u.uy); /* update position */
     if (otmp->cursed) {
         pline("For some reason, you feel your presence is known.");

src/zap.c

@@ -2707,11 +2707,7 @@ zapyourself(struct obj *obj, boolean ordinary)
             You_feel("rather itchy under %s.", yname(uarmc));
             break;
         }
-        if (ordinary || !rn2(10)) { /* permanent */
-            HInvis |= FROMOUTSIDE;
-        } else { /* temporary */
-            incr_itimeout(&HInvis, d(obj->spe, 250));
-        }
+        incr_itimeout(&HInvis, rn1(15, 31));
         if (msg) {
             learn_it = TRUE;
             newsym(u.ux, u.uy);