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/apply.c

@@ -2683,7 +2683,7 @@ use_stone(struct obj *tstone)
 
     /* in case it was acquired while blinded */
     if (!Blind)
-        tstone->dknown = 1;
+        observe_object(tstone);
     known = (tstone->otyp == TOUCHSTONE && tstone->dknown
               && objects[TOUCHSTONE].oc_name_known);
     Sprintf(stonebuf, "rub on the stone%s", plur(tstone->quan));