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/pickup.c

@@ -1816,7 +1816,7 @@ pickup_object(
     /* In case of auto-pickup, where we haven't had a chance
        to look at it yet; affects docall(SCR_SCARE_MONSTER). */
     if (!Blind)
-        obj->dknown = 1;
+        observe_object(obj);
 
     if (obj == uchain) { /* do not pick up attached chain */
         return 0;