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/quest.c

@@ -127,7 +127,7 @@ artitouch(struct obj *obj)
     if (!Qstat(touched_artifact)) {
         /* in case we haven't seen the item yet (ie, currently blinded),
            this quest message describes it by name so mark it as seen */
-        obj->dknown = 1;
+        observe_object(obj);
         /* only give this message once */
         Qstat(touched_artifact) = TRUE;
         qt_pager("gotit");