Blessed potion of see invisible

Blessed potion of see invisible was guaranteed to give see invisible
intrinsic, making it far too easy to acquire.  It now has 1/10 chance
of giving it permanently, somewhat similarly to potion of invisibility.

src/potion.c

@@ -863,7 +863,7 @@ peffect_see_invisible(struct obj *otmp)
          */
         make_blinded(0L, TRUE);
     }
-    if (otmp->blessed)
+    if (otmp->blessed && !rn2(10))
         HSee_invisible |= FROMOUTSIDE;
     else
         incr_itimeout(&HSee_invisible, rn1(100, 750));