Accurately track which items have been discovered, even if not #named

This fixes a couple of bugs: a long-standing bug in which writing a
scroll by label could fail even if you've already seen a scroll with
that label (due to the game not tracking whether or not you've seen a
scroll if it doesn't have a name); and a somewhat newer bug in which
spellbooks auto-identified by Wizard knowledge were marked as having
been encountered (rather than as known but not encountered).

Breaks save file compatibility, but not bones files.

src/do_wear.c

@@ -1196,12 +1196,12 @@ learnring(struct obj *ring, boolean observed)
            mark this ring as having been seen (no need for makeknown);
            otherwise if we have seen this ring, discover its type */
         if (objects[ringtype].oc_name_known)
-            ring->dknown = 1;
+            observe_object(ring);
         else if (ring->dknown)
             makeknown(ringtype);
 #if 0 /* see learnwand() */
         else
-            ring->eknown = 1;
+            observe_object(ring);
 #endif
     }